Goodrow, S.M.; Procopio, N.A.; Korn, L.; Morton, P.; Schuster, R.; Pang, H.; Kunz, C.; Ingelido, P., and Heddendorf, B., 2017. Long-term temporal water-quality trends within the Barnegat Bay watershed, New Jersey. In: Buchanan, G.A.; Belton, T.J., and Paudel, B. (eds.), A Comprehensive Assessment of Barnegat Bay–Little Egg Harbor, New Jersey.
This regional water-quality assessment compiles and analyzes water-quality data within the Barnegat Bay watershed (Ocean County, New Jersey) to determine if significant changes and trends occurred over decadal spans. Data evaluated spanned from the 1970s through July of 2013. Trends were evaluated after regionalizing sampling locations into 17 geographic zones. Over 1700 sampling stations within freshwater and Bay zones provided over 280,000 data results for 20 parameters. The parameters evaluated include: temperature, salinity, dissolved oxygen (DO), pH, nutrients, indicator bacteria, and solids. Temperature data comparing the 1970s with the present show statistically significant increasing trends in 13 of 17 zones during the summer months. Seven of those zones also saw increasing temperatures in at least one other season. Increasing trends were also seen for salinity in 8 of the 10 estuarine zones for two or more seasons. Potentially related to the increasing trends in salinity and temperature, DO concentrations were often decreasing or unchanged. Significant decreases in DO concentration can be seen in 12 of the 17 zones for one or more seasons. The pH was shown to be increasing in 10 of the 17 zones during one to four seasons. Four zones experienced a decreasing trend in pH, but only during one to two seasons. The interpretation of the nutrient data over this long time period proved to be a challenge, with multiple species of nitrogen and phosphorus reported but often with insufficient data to draw significant conclusions. Where data were sufficient, nutrient results were mixed, with many zones showing no changes in trends.
The natural systems of the Barnegat Bay support a multitude of environmentally sensitive habitats, including shellfish beds, waterfowl nesting grounds, submerged aquatic vegetation, bay islands and finfish nurseries. Although over 65% of the freshwater area of the 1709 km2 (660 mi2) Barnegat Bay watershed is in the Pinelands National Reserve, which has strict development regulations, the remaining area has undergone a dramatic increase in population and the stressors that accompany population increases. Over the past several decades, the population in Ocean County has increased over 175% from 208,470 in 1970 to 576,567 by 2010 (U.S. Census Bureau, 2010). Likewise, land-use changes between 1986 and 2012 (NJDEP Bureau of Geographic Information System, 2016) reveals an increase in urban land of 12.4% and 14.1% in the Toms River and Metedeconk River watersheds, respectively. Percentage increases in urban land among the other freshwater watersheds evaluated herein ranged from 0.3% to 9.4%.
Other important changes include the closing of 42 small “package” wastewater treatment plants that formerly discharged their treated wastewater to the Bay and freshwater portions of the estuary. This treated water originated as freshwater obtained through surface-water intakes or groundwater pumping for residential and commercial use. To protect surface water and groundwater quality, treated wastewater was discharged farther from the shore, and by 1979, the last of these package plants closed and wastewater was redirected to regional treatment plants. Three outfalls from these regionalized wastewater treatment plants now bypass the estuary and discharge directly to the ocean, thereby effectively reducing freshwater input to the bay. This extraction of freshwater resources has likely reduced base flows and overall stream flows in parts of the region.
Dow (2007) evaluated changes in stream flow in the Pinelands National Reserve area and reported a decrease in base flow of 3% over the period 1929–2001 for the Toms River. The Great Egg Harbor River, about 70 km to the SW of the Toms River station (40 km from the southern part of the watershed), showed a similar decrease in base flow, −1.9% over its period of record (1927–2001). Other regional sites were shown to have no change or an increase in base flow over varying periods. Additionally, Watson et al. (2005) document that high and low 1-, 7-, and 30-day flow statistics showed no significant trends for the period 1966–2001 in the Toms River and the north branch of the Metedeconk River. Both rivers are in the northern reaches of the Barnegat Bay watershed. Although insightful, the noted changes in stream flow are characteristic to unique sites and do not account for changes in adjacent watersheds or direct groundwater discharge to the Bay.
Data used for the water-quality analysis were collected between January 1970 and July 2013. Given these boundaries, the temporal trends, whether statistically increasing, decreasing, or not present, were determined for those areas where sufficient data for the statistical analysis were available.
Changes in water quality over long periods of time are evaluated to assess the interdecadal water-quality conditions of Barnegat Bay, in New Jersey. To that end, an in-depth step-trend analysis was performed using all relevant water-quality parameters over the longest time period available. The results of these analyses provide useful information regarding the status of the Bay and how the natural system has been affected by alterations in the environment over time. The results are intended to be reliable indicators of change that will inform watershed managers and the public alike.
Trend Analysis Overview
The following step-trend analysis was performed on the basis of a significant amount of water-quality data collected within the boundaries of the Barnegat Bay watershed. Sufficient water-quality data started to become available in the 1970s, making this time period the earliest boundary. If there were insufficient data to perform prescribed statistical analyses, it has been noted. Data from monitoring sites were grouped and analyzed by geographically defined zones with the intention of representing the trends of areas with similar characteristics. This approach differs from the more traditional site-by-site analyses, but is intended to provide a broad, regionalized view of the water-quality trends this area is experiencing. Results can be used in conjunction with long-term monotonic trend analyses evaluated at only one or two monitoring sites in the Bay region (Hickman and Barringer, 1999; Hickman and Gray, 2010).
Data compiled for this analysis were extracted from three databases: the U.S. Geological Survey's National Water Information System, the U.S. Environmental Protection Agency's (EPA) STORET data warehouse, and the EPA's Legacy STORET data center. Each collection of data varied in quantity and quality and was therefore evaluated according to a strict protocol for examining data.
Multiple data-mining and reduction strategies were implemented in an effort to obtain data collected under similar circumstances and raise the level of confidence to ensure data were comparable and representative of the designated region. These strategies included limiting the data to samples taken where the water was sampled from a depth of less than 2 feet and samples were limited to only those collected between 0900 and 1100. Samples with no times or odd times (e.g., 2500, 9999, 1080) were excluded. Results were further limited to one sample per day. In the case of multiple samples taken on the same day within the specified time window, the sample taken closest to the 0900 hour was chosen. To remove obvious erroneous concentrations, upper and lower limits of acceptable values were determined for relevant parameters. Any values outside of these limits were not included in the analyses. The final data set contained just over 288,000 values representing 20 parameters. A total of 1712 unique monitoring stations with data for at least one parameter was included in this analysis. The parameters evaluated in this study, selected boundaries, and the percentage of data points excluded from analysis are summarized in Table 1.
The statistical methods used in this analysis involved a unique approach to integrate multiple sites across a broad, but similar, area. All data were compiled and assigned to one of 17 delineated zones on the basis of the location of each sampling site. In many cases, the number of data points attributed to a certain zone for a specific parameter was lower than desired to assess statistical significance.
Two important caveats apply when considering the results of this study. These caveats relate to the complex nature of long-term trend analysis for a large area such as the Barnegat Bay. First, although data may have been collected from a specific site over time, all data used for this study that originated within a delineated zone could have been collected from any area within that zone, and was not necessarily collected at any one particular site. In all instances, results from multiple stations within a single zone have been combined over the time span of a decade to represent the water quality of that zone. Second, in some cases there was a low number of samples used to represent a parameter, collected in a specific season and in a specific zone. These data may be representative of a given zone; however, statistical power to identify trends decreases as sample size decreases. Therefore, caution should be used in interpreting results where a small sample size in one or both time periods exists. As such, the results of this study can be considered a useful tool in describing potential trends in the system, but should be coordinated with other studies (Potapova et al., 2016; Ren, 2016; Taghon et al., 2016) to properly describe the conditions of the watershed.
Regional Area Designations
Data collected from the multiple monitoring sites obtained from the various databases were grouped into 17 zones of relatively uniform geographic and land-use properties. There were seven watershed zones (a.k.a. “freshwater areas”) and 10 bay zones (a.k.a. “estuarine”). The size of the zone delineations ranged from 7.3 km2 (2.8 mi2) to 347 km2 (134 mi2). Included in these 17 zones were four small areas delineated for the unique nature of their drainage area or hydrology. The size of these four zones (three estuarine and one freshwater) ranged from 7.3 km2 (2.8 mi2) to 10.4 km2 (4 mi2). Excluding these four zones, the smallest of all the zones was 31.1 km2 (12 mi2). The 17 zones and the sampling stations within the zones are depicted in Figure 1.
The seven freshwater zones were generally formed from the aggregation of multiple watershed (Hydrologic Unit Code 14) drainage areas that were similar in nature. Certain drainage areas were then combined with the adjacent small tributaries draining directly into the Bay. Attributes such as land-use characteristics and topography were used to combine these smaller delineated areas to represent larger freshwater zones of the Barnegat Bay watershed. Of these seven zones, six have an area greater than 44 square miles. One freshwater area, the “Mill, Westecunk, Tuckerton near community” is actually two separate smaller lagoonal areas with high density of developed land. The intention of removing these areas from the larger zone was to eliminate the potential effects of the intense development from masking the conditions of the larger, more undeveloped zone. Monitoring sites within these two areas were analyzed as a single unit. See Table 2 for details on the freshwater zones.
The 10 bay zones were identified and delineated after evaluating the hydrological, chemical, and biological features of the Bay on the basis of the recent years' monitoring program and ongoing model simulation completed to date (U.S. Geological Survey, 2015). A water-quality data cluster analysis conducted by the New Jersey Department of Environmental Protection (NJDEP) from 14 in-bay stations was combined with preliminary results of the Barnegat Bay hydrodynamic model that provided circulation patterns and residence times (U.S. Geological Survey, 2015) to aid in the determination of the 10 estuarine zone boundaries (Figure 1). The 10 estuarine zones included seven larger zones ranging in size from 31.1 km2 (12 mi2) to 67.3 km2 (26 mi2) and three smaller zones ranging in size from 8.3 km2 (3.2 mi2) to 10.4 km2 (4 mi2) (Table 3). One of these smaller units, the Oyster Creek Tributaries–Cooling Channel area was delineated to assess the possible effects of the Oyster Creek Nuclear Generating Station on the water quality.
For the purposes of temporal trends analysis, the data were divided into groups on the basis of decade, season, and zone. The dates were grouped into respective decades (1970–79, 1980–89, 1990–99, 2000–09, and 2010–13). At the time of this analysis, data were only available up to July of 2013 and therefore data from 2010 to 2013 were treated as representative of the decade. Seasons were defined as March–May (spring), June–August (summer), September–November (fall), and December–February (winter). Because the station samples were grouped together on the basis of location and decade, there was the possibility of spatial and temporal correlations among sites within each zone. In an effort to minimize complications associated with the non-normal distribution of the data, we relied heavily on nonparametric statistical approaches that do not have the same assumptions as traditional parametric tests.
There were two types of censoring that occurred in the data. Left censoring occurred when data were reported below the method detection limits (MDL) and right censoring occurred with the coliform data when the counts were too numerous to count. Several parameters had no censoring requirements. These parameters included dissolved oxygen (DO), DO saturation, specific conductance, temperature, and pH. Fecal coliform and total coliform had both left and right censoring present in the data. All of the other parameters included in this report were subject to left censoring.
The methods used for testing differences between decades depended on the parameter and type of censoring used. When censoring was not present, the Wilcoxon rank sum (Mann-Whitney) test (Helsel and Hirsch, 2002; Hollander, Wolfe, and Chicken, 2014) was used to test for differences between decades. This is a nonparametric test that converts the data to ranks. The lowest value is assigned a rank of 1. If there are no tied values, the second lowest value gets a rank of 2. This process continues until the maximum value is reached. If there are tied values, the average rank for the group of tied values is used. The sum of the ranks in each decade is compared for testing differences. Since ranks are used rather than actual concentrations, the influence of outliers is greatly reduced and distributional assumptions are not necessary.
When censoring was present, the rank ordering system used by the Wilcoxon test may not work properly. If the MDL of a nondetect sample is larger than a sample with a quantified or other left-censored value, then the relative order of these two samples cannot be conclusively determined. Many methods have been developed to handle this complication (Helsel, 2012). The traditional method of substituting a value such as one-half of the detection limit for a nondetect has been shown to perform poorly in most situations (Helsel, 2012). In this report, when left censoring was present and no right censoring was present in a parameter, the log-rank (Mantel's) test (Hollander, Wolfe, and Chicken, 2014) was used to test differences between decades. The log-rank test is also a nonparametric test based on ranks. It was originally developed for right-censored data, so a simple adjustment (subtracting each value from a large constant) is done to make it appropriate for left-censored data. When there are both left- and right-censored data present, it is not possible to do a nonparametric test. For this case a regression model was fit by maximum likelihood with decade as an independent variable, assuming a normal distribution of the error terms with the log (base 10) bacterial count as the dependent variable. A chi-square test was then performed to test whether decade was a significant effect. All tests were done using SASTM software, version 9.3 (SAS Institute Inc., 2008).
Tables of summary statistics, by zones, are presented in the Supplementary Materials A. The types of censoring present for each parameter will affect the related summary statistics. For instance, no minimum value can be determined for left-censored data. In addition, the median, minimum, and maximum values cannot always be accurately determined for the right- and left-censored data.
Tables of decadal comparisons are presented in Supplementary Material B. Tests comparing the seasonal data of each decade with the most recent period (2010–2013) of record were referred to as primary tests and are provided in Supplementary Material B (primary decade comparisons). Tests on seasonal data between adjacent decades were referred to as secondary tests and are provided in the second table in Supplementary Material B (secondary decade comparisons). These tables have been organized by the delineated freshwater zones or estuary zones. The test result of each comparison is either −1, 0, 1, or .. −1 indicates a significant decrease from the earlier decade. +1 indicates a significant increase from the earlier decade. A 0 indicates a nonsignificant test result, whereas . indicates that no test could be performed because of inadequate sample size or too many censored values. Significance was assessed at the 0.05 confidence level.
The results of these trend analyses include seasonal and zone-specific patterns as well as decadal comparisons. As previously noted, certain parameters had a greater number of valid data points available for analysis. Since no methodology could be applied to ascertain the appropriate number of values that should be used for this analysis, a qualitative assessment has been applied to parameters on the basis of the number of input data values within zones per season. The results of trend analyses for temperature, salinity, DO, and pH discussed below are considered to be based on an adequate number of data points. The nutrient analysis includes multiple parameters of various species reported over the years. These parameters often had a low sample size. As such, there is less confidence in the results of statistical analysis.
Analysis of six of the seven freshwater zones (excluding the Mill, Westecunk, Tuckerton near-community zone, due to insufficient data for analysis) within the Barnegat Bay freshwater system for the period comparing the 1970s to the present time indicated that all zones showed no significant trend in temperature change during the spring season (see Supplementary Material B). However, the data do suggest a significant rise in temperature in four of these six zones during the summer months. That rising trend continues into the fall season in the northern three zones and into the winter months in the northern three zones and the southernmost zone (Figure 2).
The primary decadal comparisons present the largest number of instances where the trend analysis shows increasing temperatures in these freshwaters, with 11 results over six basins and four seasons indicating an increasing trend in temperature. When comparing the results between the 1980s and the present period, four of the six major freshwater zones increased in temperature for at least two seasons, with nine results indicating statistical significance. Among all of the freshwater zones, the Metedeconk River and Lower Tributaries zone included the greatest number of results that indicate an increase in the water temperature.
In the Bay zones, the comparison of temperature data from the 1970s with data collected between 2010 and 2013 (present time) shows an increasing trend across many zones (Figure 2). Comparing data from the summer season only, nine of the 10 Bay subsections indicate a statistically significant increasing trend, with only Oyster Creek Tributaries–Cooling Channel, which experiences temperature directly related to the operation of the nuclear power plant, showing a declining trend in temperature. During the spring, fall, and winter seasons, many subsections are determined to either have significant increases or no determined increase in temperature. The results for the Central Bay West zone show statistically significant increasing trends for the temperature across all four seasons. Similar trends are seen in the other primary decade comparisons, until the last comparison, 2000s to the present time, where Manahawkin Bay and Upper Little Egg Harbor, along with Oyster Creek Tributaries–Cooling Channel report two and one seasons, respectively, of decreasing trends.
The bay zones contained sufficient data for input to the trend analysis. The trend of salinity from the decade of the 1970s to 2010–13 shows a statistically significant increase across many of the estuarine zones of Barnegat Bay. Trends evaluated during the summer and fall seasons indicate a statistically significant increase in salinity throughout nine of the 10 estuarine zones of the Barnegat Bay watershed (Figure 3). The region of the Metedeconk Estuary showed no significant trend. Evaluating the seasonal data for the spring, only the southernmost two Bay regions, along with the Oyster Creek Tributaries–Cooling Channel zone in the central part of the Bay and the Metedeconk Estuary in the north, were found to have no significant change in salinity. During the winter, only two zones, Manahawkin Bay/Upper Little Egg Harbor and the Metedeconk Estuary, were found to have no statistically significant trend in salinity.
During the spring and summer seasons, the six major freshwater zones show a mix of declining trends or no trends in DO concentration (Figure 4) and DO percent saturation (Figure 5). Results from the data analyzed for the Wrangle Brook zone show the most consistent decline in trends for both the DO concentration and the percent saturated DO, with all four seasons showing significantly lower concentration of DO and three of four seasons showing lower percent saturation. Only the results for the lower-most freshwater zone of the Mill, Westecunk, and Tuckerton indicate an increasing trend in percent saturation (in three of the four seasons), with no trend shown for concentration.
When evaluating only the DO concentration in the estuarine zones in the spring, the northernmost zones are seen to have a declining trend in DO concentration, whereas the middle and southern zones show no trend. In the summer, all but the Metedeconk Estuary and the Lower Little Egg Harbor Bay show a decreasing trend. In the fall, all but the Metedeconk River estuary and the Point Pleasant Canal/Bay Head Harbor show decreasing trends. All other areas show either no trend or contain insufficient data for analysis.
When reviewing the calculated trends for the percent saturation of DO in the spring, the northernmost zones appear to have no trend, but the southern regions show an increasing trend in percent saturation of DO. Analysis of the summer, fall, and winter trends in the regions shows various patterns of increasing trends and areas with insufficient data for analysis.
The trend comparison of the 1970s pH data with the present period shows that all freshwater zones show increasing or no trends in the magnitude of pH readings (Figure 6). The two northern-most delineated areas, the Metedeconk and Lower Tributaries and the Toms River zones, showed consistent increasing trends in pH across all four seasons. The southernmost zone, the Mill, Westecunk, and Tuckerton, showed increasing trends for three of the four seasons. The pH continues to show multiple zones of increasing trends when analyzing data from the 1980s, 1990s, and the 2000s, although the number of significant findings goes down.
When comparing consecutive decades (secondary decade comparison), these increasing trends in pH are particularly evident between the 1970s and the 1980s, where all freshwater zones show a minimum of two seasons of significant increases in pH.
In the Bay zones, the trend of pH from the decade of the 1970s to the present time shows a mix of decline (becoming more acidic) and increase (becoming more basic). Spring typically showed either no trend or a trend of increasing pH, whereas the summer months indicated that three areas, the Metedeconk and Lower Tributaries–Bay, the Manahawkin Bay and Upper Little Egg Harbor, and the Lower Little Egg Harbor, have declining trends in pH. The Metedeconk and Lower Tributaries–Bay continued to display a declining trend into the fall.
Nutrients (Nitrogen and Phosphorus)
In the freshwater zones, the data necessary for comparison were limited in all zones except the northernmost zones of the Metedeconk and Lower Tributaries and the Toms River zones. Looking at the trends from the 1970s, there was a statistically significant decrease in the total orthophosphate in the spring and summer and in dissolved orthophosphate in all four seasons in the Metedeconk River and Lower Tributaries. Declining trends in levels of total phosphorus were also seen in the 1970s comparison in the fall for the Metedeconk River and Lower Tributaries and in the summer and fall in the Toms River zone. The Toms River saw a declining trend in the dissolved orthophosphate for the spring, summer, and fall. These decreasing trends continue to be seen when comparing the 1980s with the present time.
Also in the 1970s comparison, localized increasing trends in dissolved orthophosphate were seen for the Wrangle Brook zone for the fall (the sole season with sufficient data). Increasing trends for total phosphorus were noted for the Metedeconk and Lower Tributaries in the winter, the Wrangle Brook for the summer, and the Mill, Westecunk, and Tuckerton in the summer (the sole season with sufficient data for analysis). Data analyzed in this study for the 1990s and 2000s indicated that the trends level off, with reports of “no trend” in orthophosphate concentrations in the Metedeconk River and Lower Tributaries zone for each season.
Localized decreases in dissolved orthophosphate can be seen in one or two seasons in six of the 10 Bay zones (Point Pleasant Canal and Bay Head Harbor in addition to five of the southernmost zones) in the study comparing the 1970s data with the present time. This same comparison indicates a significant increase in total phosphorus scattered in three seasons (spring, summer, and fall) in four zones (Point Pleasant, Metedeconk and Lower Tributaries, Central-West, and Manahawkin Bay). Most Bay zones did not possess sufficient data for the full analysis of orthophosphate across the seasons for any of the primary decadal comparisons.
The results for the comparison of the nitrogen species between the 1970s and the present time included the most complete data set for the northernmost freshwater zone, the Metedeconk and Lower Tributaries. Three seasons of four showed no trend for inorganic nitrogen (nitrate plus nitrite), Kjeldahl nitrogen, and total nitrate. The winter data indicated a statistically significant increase in inorganic nitrogen and a statistically significant decrease in Kjeldahl nitrogen. The results for the spring season showed a statistically significant decrease in nitrate. Data for these three nitrogen species in other zones were not sufficient to make conclusions, but where data were available, the results of statistical significance were mixed, with most results (33 of 39 seasonal zone trends) indicating no trend, neither increasing nor decreasing.
Upon review of the primary decade comparison results, many zones had an insufficient amount of additional nitrogen species (nitrite, nutrient-nitrogen-total, and nutrient-nitrogen-dissolved) data for comparisons to draw meaningful conclusions. Where there were sufficient data, the most prevalent result indicated that there was no significant change in concentration of the parameter. However, nutrient-nitrogen was found to be increasing between the 1980s and the present time during the spring and summer in the Metedeconk and Lower Tributaries zone and in winter in the Toms River zone. The Toms River zone also had a statistically significant increase in inorganic nitrogen in the spring and summer.
The nitrogen parameters were also analyzed as part of the secondary decade analysis (results also included in Supplementary Appendix B). An overview of these results shows that there is a mix of trend results, with no significant trend dominating the results. However, a statistically significant increase in Kjeldahl nitrogen and nitrite were found in the northernmost zone (Metedeconk and Lower Tributaries), whereas the summer nutrient-nitrogen decreased between the 1970s and the 1980s. In the next decadal comparison (1980s to the 1990s) in this zone, the Kjeldahl nitrogen decreased for three seasons and dissolved nutrient-nitrogen decreased during the fall. This analysis also indicates that the inorganic nitrogen significantly decreased in the Toms River zone for the summer, fall, and winter while comparing the 1990 data sets with data from the 2000s.
In the 1970s comparison, four Bay zones had sufficient data for total inorganic nitrogen (nitrate plus nitrite) across one to three seasons. Most results showed no trends; however, the Metedeconk and Lower Tributaries–Bay did show a statistically significant decrease in inorganic nitrogen concentrations. This decadal comparison also provided some results for total nitrogen, again showing no trends for two seasons in two zones, but decreasing trends for two seasons (summer and winter) in the Oyster Creek Tributaries–Cooling Channel and for spring in the Toms River estuary.
When comparing the 1980s with the present time, the six zones that had sufficient data during the summer season showed decreasing trends in the concentrations of total nitrogen. The secondary decadal comparison, evaluating trends between neighboring decades, indicates multiple zones of decreasing trends or no trends in various nitrogen species where data were sufficient to complete the analyses.
The analysis of each of the 17 zones of the Barnegat Bay watershed include four seasons, four primary decadal analyses, and four secondary decadal analyses on 20 parameters. The results of all comparisons can be found in Supplementary Materials B. For the four parameters (temperature, salinity, DO, and temperature) where there were adequate data across many zones and seasons, the results were summarized above.
Long-term trends of temperature are important because all biological and chemical processes are affected by temperature. All aquatic organisms are dependent on a certain range of temperatures for optimal health. The results of the analysis of the temperature data showed trends that proved noteworthy in that 14 of the 17 zones showed a statistically significant increase in temperature during the summer months. In addition, this increasing trend can be seen during other seasons in the three northern zones (Metedeconk River and Lower Tributaries, Toms River, and Wrangle Brook) as well as the most southern zone, the Little Egg Harbor.
Long-term trends of salinity are important because many aquatic organisms present in the estuary are sensitive to changes in salinity. Ecosystems can differ in basic structure according to the range of salinities that are present. A salinity gradient in the Bay is present, with the northern reaches typically less saline than the southern reaches (Taghon et al., 2016). Increasing trends in salinity were apparent in the estuarine zones where primary decadal analysis showed statistically significant increases in nine of the 10 estuarine zones during all of the summer and fall seasons, and in some zones during the spring and winter seasons.
Freshwater input to the Bay is a primary control of estuary salinity. The salinity of water within an estuary will increase with distance from the freshwater source. However, in this evaluation, it is not the spatial aspect of the magnitude of salinity that is notable, but the change over time, in the magnitude of the salinity. It is possible that the reduction of freshwater flow to the estuary is one of the drivers of increased salinity observed in some zones.
The withdrawal of freshwater from surface- and groundwater sources in Ocean County has increased from about 56 million gallons per day in 1985 to about 71 million gallons per day in 2000. The majority of the withdrawn water (70%) is for public supply. The amount of freshwater removed from the watershed through regional sewerage outfall to the ocean has been estimated to average approximately 60 million gallons per day during high-demand summer months. This volume of freshwater is equivalent to about one-third of the freshwater inflow to the estuary under extreme low-flow conditions (Barnegat Bay National Estuary Program, 2005).
Oxygen dissolved in natural water becomes consumed through biological processes, including the degradation of plant and animal material. As more nutrients enter the system, the balance of oxygen consumption and reaeration can become unsustainable, with oxygen consumption dominating the system. In addition, the rates of reaeration can be affected by altered bathymetry and reduced flows. However, in the case of Barnegat Bay, the trends in DO concentration and DO saturation need to be evaluated along with changes in temperature and salinity to determine the true extent of the impact of biological processes.
The trends in the DO concentration and the DO saturation levels are important because of the potential impact on the biota within the estuary. Warmer and more saline waters hold lower amounts of DO and the percent saturation is based on the maximum amount of oxygen that waters with a specific temperature and salinity can potentially contain. Therefore, it is possible that the percent saturation remains stable as concentration declines if temperature, salinity, or a combination of the two increases concurrently. In the current analysis, the trends for both the DO concentration and the percent saturation do not track each other as would theoretically be expected, given that there were changes in salinity and temperature over the same time period. Decreases in DO concentration (mg/L) may also be directly and only related to increases in temperature and salinity, whereas decreases in percent saturation may be caused by DO consumption and therefore have unrelated trends. In addition, other factors such as the variation that may occur with instrumentation type and calibration method and the fact that all data analyzed were not necessarily paired data (i.e. DO concentration [mg/L] and percent saturation were not measured at the same site at the same time) may also cause inconsistent trend results.
Overall, it appears that many areas are experiencing a decrease in DO. At the same time, many areas (oftentimes different areas) are experiencing increasing trends in the percent saturation of DO. Using this information for protection of biota will require a full analysis of colocated parameters and associated impacts.
The pH of estuarine waters can be altered by several mechanisms. Sea-level rise, ocean acidification, shifts in precipitation, hypoxia, along with changes in freshwater inputs could all contribute to changes in pH found in the estuary. Additionally, groundwater pumping and surface-water intakes also reduce freshwater from drainage areas that drain to the Barnegat Bay. This reduction in flow from lower-pH freshwaters may contribute to an increase in the pH measured in the receiving water body of the bay. It is likely that a variety of causes may be playing a role across the watershed, with some impacts being felt more strongly in certain areas.
Many of the streams in the Barnegat Bay region flow through the Pinelands National Reserve. This unique ecosystem is characterized by much lower pH in the freshwater systems as compared with the rest of the state. As development and alteration of the natural landscape increases, the use of lime and other chemicals associated with landscaping and farming practices, as well as assimilation and uptake of nutrients by aquatic plants (Brewer and Goldman 1976; Schindler, Turner, and Hesslein, 1985), can contribute to the increase in pH in the waterways. A study of the Barnegat Bay watershed performed by the Pinelands Commission demonstrated that both pH and specific conductance tend to increase as the percentage of altered land in a drainage basin increases (Zampella et al., 2006).
The Toms River zone, which is the most developed of the freshwater zones within the Pinelands Management Area, exemplifies this observation. A significant increase from low pH values to higher pH values, uncharacteristic to the Pinelands waters, was observed in each season. The southernmost freshwater zone, the Mill Creek, Westecunk Creek, and Tuckerton Creek zone, which is dominated by the largely undeveloped Forest and Preservation Areas of the Pinelands Reserve, also showed a significant deviance from typically low pH values to borderline uncharacteristic values exceeding 5.5. The Cedar Creek and Oyster Creek, Forked River, Waretown Creek, and Lower Tributary zones, two zones primarily within Forest and Preservation Areas, continue to retain characteristically low pH and revealed no significant changes in pH since the 1970s. The Wrangle Brook zone, with a mix of designated Pinelands growth and protection areas, showed a significant increase in pH in the summer, but the median value for the summer in the 2010–2013 period is still characteristically low.
Although the reduction in freshwater inputs to the Bay is substantial, there are other factors that may be considered causal in this increasing trend. Denitrification and sulfate reduction in a hypoxic environment, photosynthesis and respiration, sea-level rise, shifts in precipitation, and changes in carbonate chemistry can all influence pH.
All freshwater zones incurred increasing or no trends in the level of pH, with the Metedeconk and Toms River zones showing a consistent increasing trend in pH. In areas where groundwater extraction is more abundant, an increase in the pumping of freshwater from public and private wells may decrease the stream flows of naturally acidic waters, thereby increasing the pH of some of the receiving Bay waters. This increase is most notable during the spring and winter months. However, during the summer, in four of the estuarine zones (Point Pleasant Canal and Bay Head Harbor, Metedeconk Bay, Manahawkin Bay, and Upper Little Egg Harbor and Lower Little Egg Harbor), there was a statistically significant decline in pH.
The data collected for this analysis included a variety of species that could be considered “nutrients” and potentially affect the trophic status of the water body, given the appropriate conditions. However, the statistical methods used in this study required a minimum number of values of a similar parameter type. Many of these parameters were collected and reported infrequently and so the results of this analysis are limited. Nutrients analyzed included phosphorus and nitrogen species.
Phosphorus can be found in many different forms. Soluble reactive phosphorus, also known as dissolved orthophosphate, is a form that can be readily used by phytoplankton. Particulate-bound orthophosphate needs to separate from particles for biological use. Acid hydrolyzable phosphorus, or inorganic phosphates (P2O7, P3O10), are forms typically used in fertilizers. “Total phosphorus” is a measure that includes all forms of phosphorus. The analytes of orthophosphate (total and dissolved) and total phosphorus were reviewed for this analysis because of the higher number of values available for review.
Multiple parameters representing the nitrogen concentration of the freshwaters of the Barnegat Bay watershed were reviewed for this analysis. These parameters include total inorganic nitrogen (nitrate and nitrite), total Kjeldahl nitrogen, total nitrate, total nitrite, total nitrogen, and dissolved nutrient-nitrogen. Sufficient data were not available for all parameters for all seasons for all zones, but, on the basis of the data that were available and analyzed, certain results proved to be significant.
Data collected for the nutrient series also proved challenging, due to the fact that multiple species of both nitrogen and phosphorus have been quantified and reported through the years. In many areas, long-term trends could not be determined because of the lack of sufficient data. However, even where there were sufficient data, a mixed message evolved. It appears that dissolved orthophosphate declined during the spring and summer in the Metedeconk and in the spring, summer, and fall in the Toms River zone. However, the dissolved orthophosphate concentration showed statistically significant increases in the fall in the Wrangle Brook zone and in the Mill, Westecunk, and Tuckerton zone in the summer. The nitrogen series, where sufficient data existed, largely showed no trends in regional concentrations.
The Barnegat Bay watershed and estuary has seen varied levels of water quality for many years. Land-use changes and the uses of the estuary have the potential to affect this water quality. Intensive data compilation and robust statistical analysis has provided a much-needed window into the potentially changing water quality within the region. Overall, sweeping conclusions cannot be made because of the interaction of certain water-quality parameters (e.g., DO and temperature) and caveats outlined earlier, including parameters with limited data. Despite this, several notable results highlight the areas where changes of water quality, including temperature, salinity, and pH, have occurred. The trend analysis contained in this report was able to determine, with varying levels of confidence, the direction, but not magnitude, of these trends and will be an important component of a full analysis of the health of the system.
We thank Gary Buchanan, Barbara Hirst, and Debra Hammond of the NJDEP for their guidance in direction for this effort. We also thank Theresa Tucker of the NJDEP for editorial assistance. Last, we are appreciative of the reviewers for their constructive comments.