How the Distribution of Anthropogenic Nitrogen Has Changed in Narragansett Bay (RI, USA) Following Major Reductions in Nutrient Loads

Authors

Autumn Oczkowski, Courtney Schmidt, Emily Santos, Kenneth Miller, Alana Hanson, Donald Cobb, Jason Krumholz, Adam Pimenta, Leanna Heffner, Sandra Robinson, Joaquín Chaves, Rick McKinney

Over the past decade, nitrogen (N) loads to Narragansett Bay have decreased by more than 50%. These reductions were, in large part, the direct result of multiple wastewater treatment facility upgrades to tertiary treatment, a process which employs N removal. Here, we document ecosystem response to the N reductions and assess how the distribution of sewage N in Narragansett Bay has changed from before, during, and shortly after the upgrades. While others have observed clear responses when data were considered annually, our seasonal and regional comparisons of pre- and post-tertiary treatment dissolved inorganic nitrogen (DIN) concentrations and Secchi depth data, from bay-wide surveys conducted periodically from the early 1970s through 2016, resulted in only a few subtle differences. Thus, we sought to use stable isotope data to assess how sewage N is incorporated into the ecology of the Bay and how its distribution may have changed after the upgrades. The nitrogen (δ¹⁵N) and carbon (δ¹³C) stable isotope measurements of particulate matter served as a proxy for phytoplankton, while macroalgae served as short-term integrators of water column bio-available N, and hard clams (Mercenaria mercenaria) as integrators of water column production. In contrast to other estuarine stable isotope studies that have observed an increased influence of isotopically lower marine N when sewage N is reduced, the opposite has occurred in Narragansett Bay. The tertiary treatment upgrades have increased the effluent δ¹⁵N values by at least 2‰. The plants and animals throughout Narragansett Bay have similarly increased by 1–2‰, on average. In contrast, the δ¹³C values measured in particulate matter and hard clams have declined by about the same amount. The δ¹⁵N results indicated that, even after the N reductions, sewage N still plays an important role in supporting primary and secondary production throughout the bay. However, the δ¹³C suggests that overall net production in Narragansett Bay has decreased. In the 5 years after the major wastewater treatment facilities came on-line for nutrient removal, oligotrophication has begun but sewage remains the dominant source of N to Narragansett Bay.

Effects of Local Watershed Land Use on Water Quality in Mid-Atlantic Coastal Bays and Subestuaries of the Chesapeake Bay

Authors

Thomas E. Jordan, Donald E. Weller, Carey E. Pelc

Nutrient inputs have degraded estuaries worldwide. We investigated the sources and effects of nutrient inputs by comparing water quality at shallow (< 2m deep) nearshore (within 200 m) locations in a total of 49 Chesapeake subestuaries and Mid-Atlantic coastal bays with differing local watershed land use. During July–October, concentrations of total nitrogen (TN), dissolved ammonium, dissolved inorganic N (DIN), and chlorophyll a were positively correlated with the percentages of cropland and developed land in the local watersheds. TN, DIN, and nitrate were positively correlated with the ratio of watershed area to subestuary area. Total phosphorus (TP) and dissolved phosphate increased with cropland but were not affected by developed land. The relationships among N, P, chlorophyll a, and land use suggest N limitation of chlorophyll aproduction from July–October. We compared our measurements inside the subestuaries to measurements by the Chesapeake Bay Program in adjacent estuarine waters outside the subestuaries. TP and dissolved inorganic P concentrations inside the subestuaries correlated with concentrations outside the subestuaries. However, water quality inside the subestuaries generally differed from that in adjacent estuarine waters. The concentration of nitrate was lower inside the subestuaries, while the concentrations of other forms of N, TP, and chlorophyll a were higher. This suggests that shallow nearshore waters inside the subestuaries import nitrate while exporting other forms of N as well as TP and chlorophyll a. The importance of local land use and the distinct biogeochemistry of shallow waters should be considered in managing coastal systems.

Seasonal variation in the bacterial community composition of a large estuarine reservoir and response to cyanobacterial proliferation

Authors

Zheng Xu, Jason N. Woodhouse, Shu Harn Te, Karina Yew-Hoong Gin, Yiliang He, Cong Xu, Lei Chen

This study employed high-throughput sequencing (HTS) to understand the variations in microbial community in the largest estuary reservoir located at the Yangtze River for a year. Correlations between the heterotrophic bacterial and cyanobacterial communities in the estuarine ecosystem were also investigated. Significant spatial and temporal changes were observed in the microbial community composition at all sites. These differences were mainly reflected on the variations of bacterial relative abundance. The modularity analysis on the network indicate that bacterial community response to the variations of environmental factors in the form of co-occurrence/exclusion patterns. In warm season, Synechococcus spp. being the dominant Cyanobacteria taxa exhibited high relative abundance in the reservoir. Water temperature was the critical driver for the proliferation of Synechococcus. Moreover, heterotrophic bacteria belonging to Actinobacteria, Proteobacteria (α-, β-, and γ-Proteobacteria), Bacteroidetes and Chlorobi, exhibited positive correlations with Synechococcus. The co-occurrence of these bacterial OTUs suggests that specific taxa may benefit from the proliferation of Synechococcus. In cold season, bacterial OTUs belonging to Actinobacteria and Bacteroidetes shown co-occurrence pattern with salt ions (including K+, Na+, Mg2+, Ca2+, Cl− and SO42−) inside the reservoir. In conclusion, further research is required to investigate the ecological functions of these taxa in estuarine ecosystems.

Cyanobacteria and cyanotoxins at the river-estuarine transition

Authors

Paul A. Bukaveckas, Rima Franklin, Spencer Tassone, Brendan Trache, Todd Egerton

We examined seasonal and longitudinal patterns in the occurrence of toxic cyanobacteria in the James River Estuary (Virginia). Highest chlorophyll and cyanobacteria levels were observed in the tidal freshwater segment, particularly during dry summers when freshwater replacement time was long. Cyanobacteria accounted for a small proportion of phytoplankton biomass (7–15%), and Microcystis comprised a small proportion of the cyanobacteria (<1%). Despite this, measureable levels of microcystin were commonly observed in water (>85% of samples in July, August and September), fish tissues (87% of planktivorous fishes) and shellfish (83% of individuals). Generic indicators of algal blooms (chlorophyll and algal biomass) had limited utility for predicting microcystin concentrations. However, chlorophyll was found to be a useful predictor for the probability of exceeding specific toxin thresholds. Tissue microcystin concentrations were highest in fish and shellfish collected from the tidal fresh segment, but were detectable in biota collected from the oligohaline at distances 50 km seaward.

Nitrification and denitrification in estuarine sediments with tube-dwelling benthic animals

Authors

Paula Carpintero MoraesMindaugas ZiliusSara BenelliMarco Bartoli

Burrowing benthic animals belonging to the same functional group may produce species-specific effects on microbially mediated nitrogen (N) processes depending upon different ecological traits. We investigated the effects of two tube-dwelling organisms, amphipods (Corophium insidiosum) and chironomid larvae (Chironomus plumosus), on benthic N cycling in bioturbated estuarine sediments. Aims of this work were to analyze the interactions among burrowers and N-related microbial processes in two distinct sedimentary environments colonized by benthic animals with different ecological traits. We hypothesized higher rates of nitrification and higher coupled nitrification–denitrification in sediments with C. insidiosumdue to continuous ventilation rates. We expected higher denitrification of water column nitrate in sediments with C. plumosus due to lower and intermittent ventilation activity and lower oxygen levels in burrows. To this purpose, we combined process–specific (nitrification and denitrification) with net N flux measurements in intact and reconstructed sediments. Sediments with C. insidiosum had higher rates of oxygen demand and of potential nitrification and higher concentration of pore water NH₄⁺ as compared to sediments with C. plumosus. Sediments with both species displayed comparable net N₂ fluxes, mostly sustained by respiration of water column NO₃⁻ in sediments with chironomid larvae and by NO₃⁻ produced within sediments in sediments with corophiid amphipods. Corophium insidiosum stimulated nitrification nearly 15-fold more as compared to C. plumosus. Overall, our results demonstrate that sediments with burrowing fauna may display similar rates of denitrification, but underlying mechanisms may deeply vary and be species-specific.

Coastal eutrophication and freshening: Impacts on Pseudo-nitzschia abundance and domoic acid allelopathy

Authors

Elise Van Meerssche, Dianne I. Greenfield, James L. Pinckney

Species of the diatom genus Pseudo-nitzschia can produce domoic acid (DA), a neurotoxin responsible for amnesic shellfish poisoning. This release of DA by toxic Pseudo-nitzschia species in the water column may be a way to help the cells to outcompete other phytoplankton species (i.e. allelopathy). In this study, the influence of salinity and nutrient inputs on the effect of DA on natural phytoplankton community compositions was investigated using bioassays to measure phytoplankton responses to 400 ng DA ml⁻¹. Dissolved DA (dDA) was added to natural phytoplankton assemblages from a low salinity site dominated by river runoff and from a relatively pristine, high salinity site in which nutrients were added simultaneously with dDA. At the low salinity site, the percent inhibition of cryptophytes and diatoms induced by the addition of dDA was negatively correlated with salinity. However, at the high salinity site with nutrients added, no correlation with salinity was observed. This study highlights how environmental factors such as salinity and nutrients can play important roles on toxin allelopathy by alleviating or exacerbating its effect.

Assessing phytoplankton composition and structure within micro-estuaries and micro-outlets: a community analysis approach

Authors

Tatenda Dalu, Mandla L. Magoro, Jonathan D. Tonkin, Lucienne R. D. Human, Renzo Perissinotto, Shaun H. P. Deyzel, Janine B. Adams, Alan K. Whitfield

Micro-estuaries and micro-outlets represent small coastal waterbodies that differ in their relative salinity and size, with the former being larger, more saline (mesohaline versus oligohaline), and exchanging with the sea more often than the latter. There are thousands of these waterbodies along the world’s coastline, yet few of these very small systems have been identified and studied. We investigated systematic differences between micro-estuaries and micro-outlets in terms of phytoplankton community composition, including spatio-temporal variation in both community structure and biomass (chlorophyll-a). A multivariate analysis was used to assess differences in environmental variables, biomass and phytoplankton community composition across four seasons and the two waterbody types. A total of 260 (63 families) and 244 (74 families) phytoplankton taxa were identified within the micro-estuaries and micro-outlets, respectively. Nano- and picoplankton were the dominant groups in micro-estuaries, and pico- and microplankton in micro-outlets. Micro-estuaries were rich in phytoplankton taxa representative of marine, estuarine and freshwater conditions, with a successional sequence in dominance evident, from Chlorophyta during winter to Bacillariophyta in spring and Cyanophyta in summer. By contrast, micro-outlets were mostly dominated by freshwater taxa, with Chlorophyta remaining the dominant group across all four seasons. Higher phytoplankton biomass was recorded during the winter when increased nutrients were available following catchment flooding. Seasonal switching in phytoplankton was reflected not only in changing dominance patterns in both habitat types but also in complete replacement of some species in micro-outlets, despite Chlorophyta remaining dominant. Such temporal turnover, which is often accompanied by predictable seasonal changes in environmental conditions, can promote overall species richness by allowing more taxa to coexist in a single environment through temporal niche segregation.

Quantifying Metabolically Driven pH and Oxygen Fluctuations in US Nearshore Habitats at Diel to Interannual Time Scales

Authors

Hannes Baumann, Erik M. Smith

We compiled and examined 15 years (2002–2016) of high-frequency monitoring data from the National Estuarine Research Reserve System (NERRS) to characterize diel to interannual variability of pH and dissolved oxygen (DO, % saturation) across 16 diverse, shallow-water habitats along the US Atlantic, Gulf of Mexico, Caribbean, and Pacific coasts. We asked whether these systems exhibit a common pH/DO relationship, whether there were detectable interannual trends in temperature, pH, and DO within and across systems, and how pH/DO dynamics would relate to measured levels of nutrients and chlorophyll. Our analyses confirmed that large, metabolically driven, and thus concurrent fluctuations of pH and DO are a unifying feature of nearshore habitats. Moreover, we derived well-constrained relationships that predict (i) monthly mean pH or (ii) mean diel pH fluctuations across systems based on habitat mean salinity and (i) mean DO or (ii) mean diel DO fluctuations. This suggests that common metabolic principles drive diel to seasonal pH/DO variations within as well as across a diversity of estuarine environments. Yearly pH and DO anomalies did not show monotonous trends over the study period and differed considerably between sites and regions. However, weekly anomalies of means, diel minima, and diel ranges of pH and DO changed significantly over time and were strongly correlated to temperature anomalies. These general patterns lend strong empirical support to the notion that coastal acidification—in addition to being driven by eutrophication and atmospheric CO₂ increases—is exacerbated simply by warming, likely via increasing community respiration. Nutrient and chlorophyll dynamics were inversely related in these shallow, well-mixed systems, but higher nutrient levels were still associated with lower pH and lower DO levels in most, but not all, systems. Our analyses emphasize the particular dynamics of nearshore habitats and the critical importance of NERRS and its system-wide monitoring program.

Spatiotemporal patterns of phytoplankton composition and abundance in the Maryland Coastal Bays: The influence of freshwater discharge and anthropogenic activities

Authors

Ozuem F. Oseji, Paulinus Chigbu, Efeturi Oghenekaro, Yan Waguespack, Nianhong Chen

The spatial and temporal variations in phytoplankton abundance and community structure in the northern and southern parts of the Maryland Coastal Bays (MCBs) that differ in anthropogenic activities and hydrological characteristics were studied in 2012 and 2013 using photosynthetic pigments as biomarkers. Phytoplankton pigment biomass and diversity were generally higher in the northern bays that receive high nutrient input from St. Martin River, than in the southern bays where nutrient levels were comparatively low. Sites close to the mouths of tributaries in northern and southern bays had higher nutrient levels, which favored the development of dinoflagellates, and nano- and picophytoplankton, than sites closer to the inlets. The microplankton dominated the phytoplankton community in spring (>90%) and decreased in relative abundance into fall (<60%) whereas nanoplankton peaked in summer or fall. Picoplankton relative abundance increased from late spring (<10%, March 2012 & 2013) to summer (40%, July 2012 and August 2013) and was correlated positively with NH₄⁺ and negatively with salinity. The observed spatial and seasonal patterns of phytoplankton relative abundance and diversity are likely due to changes in nutrient concentrations and ratios, driven by variations in freshwater discharge, and selective grazing of phytoplankton. Water quality management in the MCBs should continue to focus on reducing nutrient inputs into the bays.

 

Oxygen and Nutrient Exchanges at the Sediment-Water Interface: a Global Synthesis and Critique of Estuarine and Coastal Data

Authors

W. R. Boynton, M. A. C. Ceballos, E. M. Bailey, C. L. S. Hodgkins, J. L. Humphrey, J. M. Testa

Estuarine and coastal marine sediment-water fluxes are considered to be important ecological features, but a global-scale assessment has yet to be developed. Goals of this work were to assemble a global-scale database of net sediment-water flux measurements, examine measurement techniques, characterize the geographic distribution and magnitude of sediment fluxes, explore the data for controls on sediment flux magnitude, and assess the importance of sediment fluxes in ecosystem-level metabolism and primary production. We examined 480 peer-reviewed sources and found sediment flux data for 167 estuarine and coastal systems. Most measurements were made in North America, Europe, and Australia. Fluxes varied widely among systems, some by several orders of magnitude. Inter-annual variability within sites was less than an order of magnitude but time series flux data to evaluate this were rare. However, limited time series data exhibited large and rapid responses to decreased external nutrient loading rates, climate change effects (possible temperature effects), and variability in trophic conditions. Comparative analyses indicated organic matter supply to sediments set the upper limits of flux magnitude, with other factors playing secondary roles. Two metrics were developed to assess ecosystem-level importance of sediment-water fluxes. Sediments represented 30% or more of depth-integrated rates of aerobic system respiration at depths of <10 m. An annual phytoplankton production data set was used to estimate N and P demand; sediments supplied an average of 15–32% of N and 17–100% of P demand and, in some cases, was as large or larger than external nutrient inputs. The percent of demand supplied by sediments was highest in temperate latitudes and lower in high and tropical latitudes.