Deciphering patterns in whole fish nitrogen isotopes on a continental scale.

Nitrogen isotopes (δ15N) have been used as an indicator of anthropogenic nitrogen loading at local and regional scales. We examined δ15N in fish from estuaries across the continental United States. In the summer of 2015, the U.S. Environmental Protection Agency's National Coastal Condition Assessment (NCCA) collected fish in 136 coastal waterbodies throughout the United States. Whole fish were analyzed by NCCA for metals, organic contaminants, and lipids. For this study, we also analyzed these fish for isotopes of nitrogen (N). NCCA collected water quality, nutrients, chlorophyll a, and sediment chemistry at each site. We used these data, along with fish life history and watershed land use, to examine how whole fish δ15N was related to these environmental variables using random forest regression models at national and ecoregional scales. At the national scale, fish δ15N were negatively related to total N:total phosphorous (P) ratios (TN:TP) in surface water and reflected differences between the P-limited, δ15N depleted sites in the Floridian ecoregion to sites in other regions. δ15N was lower on the Atlantic relative to the Pacific coast. When considered by region, TN:TP was an important predictor of fish δ15N in 4 of 9 ecoregions, with higher δ15N observed with increasing N limitation (lower TN:TP) Fish life history was also an important predictor of fish δ15N at both the national and ecoregional scale. Whole fish δ15N was positively associated with bioaccumulative contaminants such as PCBs and mercury. Although land use was related to δ15N in fish, it was location specific. This study showed that N stable isotopes reflected ecological conditions at both regional and continental scales.

Measuring and modeling diel oxygen dynamics in a shallow hypereutrophic estuary: Implications of low oxygen exposure on aquatic life.

Hypoxia, or low dissolved oxygen (DO) is a common outcome of excess nitrogen and phosphorus delivered to coastal waterbodies. Shallow and highly productive estuaries are particularly susceptible to diel-cycling hypoxia, which can exhibit DO excursions between anoxia (DO ≤1 mg L-1) and supersaturated concentrations within a day. Shallow estuaries exhibiting diel-cycling hypoxia are understudied relative to larger and deeper estuaries, with very few mechanistic models that can predict diel oxygen dynamics. We utilized continuous monitoring data and the Coastal Generalized Ecosystem Model (CGEM) coupled with an Environmental Fluid Dynamics Code (EFDC) hydrodynamic model to simulate diel DO dynamics in Weeks Bay, AL. Low oxygen conditions ranging from anoxia to DO ≤4 mg L-1 were consistently observed and simulated in the lower water column for periods of minutes to >11 h. High frequency observations and model simulations also identified significant vertical gradients in near bottom DO that varied as much as 0.8 to 3.1 mg L-1 within 0.4 m from the bottom. This spatiotemporal variability presents unique challenges to adequately quantify DO dynamics and the potential exposure of aquatic life to low oxygen conditions. Our results demonstrate the need for detailed measurements to adequately quantify the complex DO dynamics in shallow estuaries. We also demonstrate that simulation models can be successfully applied to evaluate diel oxygen dynamics in complex estuarine environments when calibrated with fine time scale data and effective parameterization of water column and benthic metabolic processes.

Effects of Biophysical Processes on Diel-Cycling Hypoxia in a Subtropical Estuary.

In shallow estuaries, fluctuations in bottom dissolved oxygen (DO) at diel (24 h) timescales are commonly attributed to cycles of net production and respiration. However, bottom DO can also be modulated by physical processes, such as tides and wind, that vary at or near diel timescales. Here, we examine processes affecting spatiotemporal variations in diel-cycling DO in Escambia Bay, a shallow estuary along the Gulf of Mexico. We collected continuous water quality measurements in the upper and middle reaches of the Bay following relatively high (> 850 m3 s-1) and low (< 175 m3 s-1) springtime freshwater discharge. Variations in diel-cycling amplitude over time were estimated using the continuous wavelet transform, and correlations between DO and biophysical processes at diel timescales were examined using wavelet coherence. Our results reveal that freshwater discharge modulated inter-annual variations in the spatial extent and duration of summertime hypoxia through its effect on vertical density stratification. In the absence of strong stratification (> 15 kg m-3), vertical mixing by tropic tides and sea breeze enhanced diel fluctuations in deeper areas near the channel, while in shallower areas the largest fluctuations were associated with irradiance. Our findings suggest that processes affecting diel-cycling DO in the bottom layer can vary over a relatively short spatial extent less than 2 km and with relatively small changes in bottom elevation of 1 m or less. Implications for water quality monitoring were illustrated by subsampling DO timeseries, which demonstrates how low-frequency measurements may misrepresent water quality in estuaries where diel-cycling DO is common. In these systems, adequate assessment of hypoxia and its aquatic life impacts requires continuous measurements that capture the variation in DO at diel timescales.

Quantifying coastal ecosystem trophic state at a macroscale using a Bayesian analytical framework.

One of the goals of coastal ecological research is to describe, quantify and predict human effects on coastal ecosystems. Broad cross-systems assessments to classify ecosystem status or condition have been developed, but are not updated frequently, likely because a lot of information and effort is needed to implement them. Such assessments could be more useful if the probability of being in a class indicating status or condition could be predicted using widely available data and information, providing a useful way to interpret changes in underlying predictors by considering their expected impact on ecosystem condition. To illustrate a possible approach, we used chlorophyll-a as an indicator of condition, in place of the intended comprehensive condition assessment. We demonstrated a predictive approach starting with a random forest model to inform variable selection, then used a Bayesian multilevel ordered categorical regression to quantify a coastal trophic state index and predict system status. We initially fit the model using non-informative priors to water quality data (total nitrogen and phosphorus, dissolved inorganic nitrogen and phosphorus, secchi depth) from 2010 and a regional factor. We then updated the model using prior distributions based on posterior parameter distributions from the initial fit and data from 2015. The Bayesian model demonstrates an intuitive way to update a model or analysis with new data while retaining the benefit of prior knowledge and maintaining flexibility to consider new kinds of information. To illustrate how the model could be used, we applied our developed trophic state index and classification to a time series of water quality data from Boston Harbor, a coastal ecosystem that has undergone significant changes in nutrient inputs. The analysis shows how water quality status and trends in Boston Harbor can be understood in the comparative ecological context provided by data from estuaries around the continental US and illustrates how the analytical approach could be used as an interpretive tool by non-practitioners of Bayesian statistics as well as a framework for further model development and analysis.

Modeling Spatiotemporal Patterns of Ecosystem Metabolism and Organic Carbon Dynamics Affecting Hypoxia on the Louisiana Continental Shelf.

The hypoxic zone on the Louisiana Continental Shelf (LCS) forms each summer due to nutrient enhanced primary production and seasonal stratification associated with freshwater discharges from the Mississippi/Atchafalaya River Basin (MARB). Recent field studies have identified highly productive shallow nearshore waters as an important component of shelf-wide carbon production contributing to hypoxia formation. In this study we present results from a three-dimensional hydrodynamic-biogeochemical model named CGEM (Coastal Generalized Ecosystem Model) applied to quantify the spatial and temporal patterns of hypoxia, carbon production, respiration, and transport between nearshore and middle shelf regions where hypoxia is most prevalent. We first demonstrate that our simulations successfully reproduced spatial and temporal patterns of carbon production, respiration, and bottom-water oxygen gradients compared to field observations. We then used interannual simulations to identify transport of particulate organic carbon (POC) from nearshore areas where riverine organic matter and phytoplankton carbon production are greatest. The spatial disconnect between carbon production and respiration in our simulations was driven by westward and offshore POC flux, a pattern that supported heterotrophic respiration on the middle shelf where hypoxia is frequently observed. These results validate the importance of offshore carbon flux to hypoxia formation, particularly on the west shelf where hypoxic conditions are more variable.

Bioassessment of a Northwest Florida Estuary Using Benthic Macroinvertebrates.

Benthic invertebrate community composition was surveyed across the salinity gradient of the Pensacola Bay Estuary in Florida during summer 2016. Macrofauna densities ranged from 1000 to 9300 individuals m-2 , with highest densities occurring at the upper estuary and the lowest in the mid- and lower estuary. Taxonomic richness and Shannon diversity were lowest in the upper estuary and increased along the salinity gradient. Small-bodied, near-surface infaunal polychaete species (e.g., Mediomastus ambiseta and Paraprionospio alata) dominated the macrofaunal community in fine sediment areas. We calculated the Gulf of Mexico Benthic Index of Biological Integrity for each site and compared the index scores with those from Environmental Monitoring and Assessment Program - Estuaries, an earlier benthic assessment model. Condition evaluations by the different models did not match across all sites in this study; however, scores consistently indicated that most sites were at or near degraded levels, implying that Pensacola Bay represents a marginal habitat for a "healthy" benthic macrofauna community. This study provided new information about the benthic communities and sediments in the Pensacola Bay estuary. Integr Environ Assess Manag 2020;16:245-256. Published 2019. This article is a US Government work and is in the public domain in the USA.

Quantifying seagrass light requirements using an algorithm to spatially resolve depth of colonization.

Depth of colonization (Zc ) is a useful seagrass growth metric that describes seagrass response to light availability. Similarly, percent surface irradiance at Zc (% SI) is an indicator of seagrass light requirements with applications in seagrass ecology and management. Methods for estimating Zc and % SI are highly variable making meaningful comparisons difficult. A new algorithm is presented to compute maps of median and maximum Zc , Zc, med and Zc,max , respectively, for four Florida coastal areas (Big Bend, Tampa Bay, Choctawhatchee Bay, Indian River Lagoon). Maps of light attenuation (Kd ) based on MODIS satellite imagery, PAR profiles, and Secchi depth measurements were combined with seagrass growth estimates to produce maps of % SI at Zc,med and Zc,max . Among estuary segments, mean Zc,med varied from (±s.e.) 0.80±0.13 m for Old Tampa Bay to 2.33±0.26 m for Western Choctawhatchee Bay. Standard errors for Zc,med were 1-10% of the segment means. Percent SI at Zc,med averaged 18% for Indian River Lagoon (range = 9-24%), 42% for Tampa Bay (37-48%) and 58% for Choctawhatchee Bay (51-75%). Estimates of % SI were significantly lower in Indian River Lagoon than in the other estuaries, while estimates for Tampa Bay and Choctawhatchee Bay were higher than the often cited estimate of 20%. Spatial gradients in depth of colonization and % SI were apparent in all estuaries. The analytical approach could be applied easily to new data from these estuaries or to other estuaries and could be incorporated routinely in assessments of seagrass status and condition.

Seasonal oxygen dynamics in a warm temperate estuary: effects of hydrologic variability on measurements of primary production, respiration, and net metabolism.

Seasonal responses in estuarine metabolism (primary production, respiration, and net metabolism) were examined using two complementary approaches. Total ecosystem metabolism rates were calculated from dissolved oxygen time series using Odum's open water method. Water column rates were calculated from oxygen-based bottle experiments. The study was conducted over a spring-summer season in the Pensacola Bay estuary at a shallow seagrass-dominated site and a deeper bare-bottomed site. Water column integrated gross production rates more than doubled (58.7 to 130.9 mmol O2 m-2 d-1) from spring to summer, coinciding with a sharp increase in water column chlorophyll-a, and a decrease in surface salinity. As expected, ecosystem gross production rates were consistently higher than water column rates, but showed a different spring-summer pattern, decreasing at the shoal site from 197 to 168 mmol O2 m-2 d-1 and sharply increasing at the channel site from 93.4 to 197.4 mmol O2 m-2 d-1. The consistency among approaches was evaluated by calculating residual metabolism rates (ecosystem - water column). At the shoal site, residual gross production rates decreased from spring to summer from 176.8 to 99.1 mmol O2 m-2 d-1, but were generally consistent with expectations for seagrass environments, indicating that the open water method captured both water column and benthic processes. However, at the channel site, where benthic production was strongly light-limited, residual gross production varied from 15.7 mmol O2 m-2 d-1 in spring to 86.7 mmol O2 m-2 d-1 in summer. The summer rates were much higher than could be realistically attributed to benthic processes, and likely reflected a violation of the open water method due to water column stratification. While the use of sensors for estimating complex ecosystem processes holds promise for coastal monitoring programs, careful attention to the sampling design, and to the underlying assumptions of the methods, is critical for correctly interpreting the results. This study demonstrated how using a combination of approaches yielded a fuller understanding of the ecosystem response to hydrologic and seasonal variability.

Novel Analyses of Long-Term Data Provide a Scientific Basis for Chlorophyll-a Thresholds in San Francisco Bay.

San Francisco Bay (SFB), USA, is highly enriched in nitrogen and phosphorus, but has been resistant to the classic symptoms of eutrophication associated with over-production of phytoplankton. Observations in recent years suggest that this resistance may be weakening, shown by: significant increases of chlorophyll-a (chl-a) and decreases of dissolved oxygen (DO), common occurrences of phytoplankton taxa that can form Harmful Algal Blooms (HAB), and algal toxins in water and mussels reaching levels of concern. As a result, managers now ask: what levels of chl-a in SFB constitute tipping points of phytoplankton biomass beyond which water quality will become degraded, requiring significant nutrient reductions to avoid impairments? We analyzed data for DO, phytoplankton species composition, chl-a, and algal toxins to derive quantitative relationships between three indicators (HAB abundance, toxin concentrations, DO) and chl-a. Quantile regressions relating HAB abundance and DO to chl-a were significant, indicating SFB is at increased risk of adverse HAB and low DO levels if chl-a continues to increase. Conditional probability analysis (CPA) showed chl-a of 13 mg m-3 as a "protective" threshold below which probabilities for exceeding alert levels for HAB abundance and toxins were reduced. This threshold was similar to chl-a of 13 - 16 mg m-3 that would meet a SFB-wide 80 % saturation Water Quality Criterion (WQC) for DO. Higher "at risk" chl-a thresholds from 25 - 40 mg m-3 corresponded to 0.5 probability of exceeding alert levels for HAB abundance, and for DO below a WQC of 5.0 mg L-1 designated for lower South Bay (LSB) and South Bay (SB). We submit these thresholds as a basis to assess eutrophication status of SFB and to inform nutrient management actions. This approach is transferrable to other estuaries to derive chl-a thresholds protective against eutrophication.

Attributes of successful actions to restore lakes and estuaries degraded by nutrient pollution.

As more success is achieved in restoring lakes and estuaries from the impacts of nutrient pollution, there is increased opportunity to evaluate the scientific, social, and policy factors associated with achieving restoration goals. We examined case studies where deliberate actions to reduce nutrient pollution and restore ecosystems resulted in ecological recovery. Prospective cases were identified from scientific literature and technical documents for lakes and estuaries with: (1) scientific evidence of nutrient pollution; (2) restoration actions taken to mitigate nutrient pollution; and (3) documented ecological improvement. Using these criteria, we identified 9 estuaries and 7 lakes spanning countries, climatic regions, physical types, depths, and watershed areas. Among 16 case studies ultimately included, 8 achieved improvements short of stated restoration goals. Five more were successful initially, but condition subsequently declined. Three of the case studies achieved their goals fully and are currently managing to maintain the restored condition. We examined each case to identify both common attributes of nutrient management, grouped into 'themes', and variations on those attributes, which were coded into categorical variables based on thorough review of documents associated with each case. The themes and variables were organized into a broad conceptual model illustrating how they relate to each other and to nutrient management outcomes. We then explored relationships among the themes and variables using multiple correspondence analysis (MCA). Results of the MCA suggested that the attributes most associated with achieving restoration goals include: (1) leadership by a dedicated watershed management agency; (2) governance through a bottom-up collaborative process; (3) a strategy that set numeric targets based on a specific ecological goal; and (4) actions to reduce nutrient loads from all sources. While our study did not provide a comprehensive road map to successful nutrient management, it suggested attributes that could be emulated in future efforts. The quantitative approach that was applied could be used to provide ongoing analysis as new examples of nutrient management success emerge.

An approach to developing numeric water quality criteria for coastal waters using the SeaWiFS Satellite Data Record.

Human activities on land increase nutrient loads to coastal waters, which can increase phytoplankton production and biomass and associated ecological impacts. Numeric nutrient water quality standards are needed to protect coastal waters from eutrophication impacts. The Environmental Protection Agency determined that numeric nutrient criteria were necessary to protect designated uses of Florida's waters. The objective of this study was to evaluate a reference condition approach for developing numeric water quality criteria for coastal waters, using data from Florida. Florida's coastal waters have not been monitored comprehensively via field sampling to support numeric criteria development. However, satellite remote sensing had the potential to provide adequate data. Spatial and temporal measures of SeaWiFS OC4 chlorophyll-a (Chl(RS)-a, mg m(-3)) were resolved across Florida's coastal waters between 1997 and 2010 and compared with in situ measurements. Statistical distributions of Chl(RS)-a were evaluated to determine a quantitative reference baseline. A binomial approach was implemented to consider how new data could be assessed against the criteria. The proposed satellite remote sensing approach to derive numeric criteria may be generally applicable to other coastal waters.

Multiple regression models for hindcasting and forecasting midsummer hypoxia in the Gulf of Mexico.

A new suite of multiple regression models was developed that describes relationships between the area of bottom water hypoxia along the northern Gulf of Mexico and Mississippi-Atchafalaya River nitrate concentration, total phosphorus (TP) concentration, and discharge. Model input variables were derived from two load estimation methods, the adjusted maximum likelihood estimation (AMLE) and the composite (COMP) method, developed by the U.S. Geological Survey. Variability in midsummer hypoxic area was described by models that incorporated May discharge, May nitrate, and February TP concentrations or their spring (discharge and nitrate) and winter (TP) averages. The regression models predicted the observed hypoxic area within +/-30%, yet model residuals showed an increasing trend with time. An additional model variable, Epoch, which allowed post-1993 observations to have a different intercept than earlier observations, suggested that hypoxic area has been 6450 km2 greater per unit discharge and nutrients since 1993. Model forecasts predicted that a dual 45% reduction in nitrate and TP concentration would likely reduce hypoxic area to approximately 5000 km2, the coastal goal established by the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force. However, the COMP load estimation method, which is more accurate than the AMLE method, resulted in a smaller predicted hypoxia response to any given nutrient reduction than models based on the AMLE method. Monte Carlo simulations predicted that five years after an instantaneous 50% nitrate reduction or dual 45% nitrate and TP reduction it would be possible to resolve a significant reduction in hypoxic area. However, if nutrient reduction targets were achieved gradually (e.g., over 10 years), much more than a decade would be required before a significant downward trend in both nutrient concentrations and hypoxic area could be resolved against the large background of interannual variability. The multiple regression models and statistical approaches applied provide improved capabilities for evaluating dual nutrient management strategies to address Gulf hypoxia and a clearer perspective on the strengths and limitations of approaching the problem using regression models.

Measured concentrations of herbicides and model predictions of atrazine fate in the Patuxent River estuary.

The environmental fate of herbicides in estuaries is poorly understood. Estuarine physical transport processes and the episodic nature of herbicide release into surface waters complicate interpretation of water concentration measurements and allocation of sources. Water concentrations of herbicides and two triazine degradation products (CIAT [6-amino-2-chloro-4-isopropylamino-s-triazine] and CEAT [6-amino-2-chloro-4-ethylamino-s-triazine]) were measured in surface water from four sites on 40 d from 4 Apr. through 29 July 19% in the Patuxent River estuary, part of the Chesapeake Bay system. Atrazine (2-chloro-4-ethylamino-6-isopropylamino-1,3,5-triazine) was most persistent and present in the highest concentrations (maximum = 1.29 microg/L). Metolachlor [2-chloro-6'-ethyl-N-(2-methoxy-1-methylethyl)-o-acetoluidide], CIAT, CEAT, and simazine (1-chloro-3,5-bisethylamino-2,4,6-triazine) were frequently detected with maximum concentration values of 0.61, 1.1, 0.76, and 0.49 microg/L, respectively. A physical transport model was used to interpret atrazine concentrations in the context of estuarine water transport, giving estimates of in situ degradation rates and total transport. The estimated half-life of atrazine in the turbid, shallow upper estuary was t(1/2) = 20 d, but was much longer (t(1/2) = 100 d) in the deeper lower estuary. Although most (93%) atrazine entered the estuary upstream via the river, simulations suggested additional inputs directly to the lower estuary. The total atrazine load to the estuary from 5 April to 15 July was 71 kg with 48% loss by degradation and 31% exported to the Chesapeake Bay. Atrazine persistence in the estuary is directly related to river flows into the estuary. Low flows will increase atrazine residence time in the upper estuary and increase degradation losses.