Anthropogenic Nutrient Loading Research Articles

Assessing human impact on Rostherne Mere, UK, using the geochemistry of organic matter

This study investigates recent changes in the geochemistry of organic material from a hypereutrophic lake (Rostherne Mere, United Kingdom) using the geochemical and molecular composition of radiometrically dated sediment cores. Modern samples suggest that recent sedimentation is dominated by algal production; however, a minor component of allochthonous organic material is present. Sediment cores reveal that absolute proxy values and the magnitude of observed changes are broadly homogenous across the lake basin. A transition to environmental conditions favouring enhanced algal productivity in recent sediments is suggested by higher total organic carbon (TOC) and lower carbon to nitrogen ratio (C/N), carbon isotope composition of organic matter (δ13Corg), and average n-alkane chain length. A strong covariance between TOC and Rock-Eval Hydrogen Index implies this transition is driven by an increasing algal contribution rather than being a response to variations in the source of organic matter. Decadal trends and abrupt shifts in organic geochemical proxies are suggested to be directly related to changes in external anthropogenic nutrient loading following the construction and decommissioning of sewage treatment plants. The development of hypereutrophic conditions likely occurred in stages, where rapid transitions are associated with the commencement of sewage effluent input in the 1930s, population increases in the 1980s, and a dramatic reduction in external nutrient loads in the 1990s. Recovery of the lake ecosystem is limited by internal nutrient recycling, and organic proxies indicate that the geochemistry of sediments has remained relatively constant since effluent diversion. This study highlights the utility of organic geochemical parameters in tracing recent eutrophication processes in lakes to provide evidence for the timing and scale of anthropogenic environmental change.

Millennial-scale decline in coho salmon abundance since the middle Holocene in a coastal Oregon watershed, USA

ABSTRACTThe population size of anadromous salmon in the Pacific Northwest is strongly influenced by decadal variation in watershed and oceanographic conditions and therefore should also be influenced by larger magnitude millennial-scale variations in these conditions. We studied δ15N of bulk organic matter in lake sediment from Woahink Lake, Oregon, as a proxy of marine-derived nutrients (MDN) from spawning coho salmon. We compared this site to a control lake with a natural barrier to salmon migration. From 7.5 to 5.4 ka, a dune was emplaced, breached, and emplaced again, forming Woahink Lake in a former estuary. δ15N decreased steadily since 5.4 ka at Woahink but not at the control lake. δ15N reached a minimum just prior to anthropogenic nutrient loading, which caused an increase in δ15N, thus precluding a comparison with the historical decline in salmon abundance. A mixing model of lake-water nitrate, developed to explore alternate scenarios for the observed range of δ15N, could not explain these results without invoking MDN input from at least several hundred salmon annually. Our results show a previously unreported pattern of a millennial-scale decline in salmon that has plausible linkages to parallel changes in ocean circulation and productivity.

Trends and seasonality of river nutrients in agricultural catchments: 18 years of weekly citizen science in France

Agriculture and urbanization have disturbed three-quarters of global ice-free land surface, delivering huge amounts of nitrogen and phosphorus to freshwater ecosystems. These excess nutrients degrade habitat and threaten human food and water security at a global scale. Because most catchments are either currently subjected to, or recovering from anthropogenic nutrient loading, understanding the short- and long-term responses of river nutrients to changes in land use is essential for effective management. We analyzed a never-published, 18-year time series of anthropogenic (NO3− and PO43−) and naturally derived (dissolved silica) riverine nutrients in 13 catchments recovering from agricultural pollution in western France. In a citizen science initiative, high-school students sampled catchments weekly, which ranged from 26 to 1489km2. Nutrient concentrations decreased substantially over the period of record (19 to 50% for NO3− and 14 to 80% for PO43−), attributable to regional, national, and international investment and regulation, which started immediately prior to monitoring. For the majority of catchments, water quality during the summer low-flow period improved faster than during winter high-flow conditions, and annual minimum concentrations improved relatively faster than annual maximum concentrations. These patterns suggest that water-quality improvements were primarily due to elimination of discrete nutrient sources with seasonally-constant discharge (e.g. human and livestock wastewater), agreeing with available land-use and municipal records. Surprisingly, long-term nutrient decreases were not accompanied by changes in nutrient seasonality in most catchments, attributable to persistent, diffuse nutrient stocks. Despite decreases, nutrient concentrations in almost all catchments remained well above eutrophication thresholds, and because additional improvements will depend on decreasing diffuse nutrient sources, future gains may be much slower than initial rate of recovery. These findings demonstrate the value of citizen science initiatives in quantifying long-term and seasonal consequences of changes in land management, which are necessary to identify sustainable limits and predict recovery timeframes.

Determination and distribution of region-specific background Secchi depth based on long-term monitoring data in the Seto Inland Sea, Japan

Secchi depth has traditionally been used to assess the light transmittance of a water body and to indicate the degree of eutrophication. Anthropogenic eutrophication can decrease Secchi depth by increasing phytoplankton density. However, Secchi depth is determined not only by phytoplankton density but also by suspended particulate matter, chromophoric dissolved organic matter, and by the optical properties of the seawater itself. These latter factors contribute to a region-specific component of the Secchi depth that is independent of phytoplankton density. The present study aims to estimate this region-specific Secchi depth component, which we term the background Secchi depth (BSD). We compared measured Secchi depth for the past 10 years against BSD to understand the optical condition of extant water bodies and the limits of anthropogenic nutrient reduction as a method of increasing the Secchi depth. Our study analyzed 34 years of seasonal monitoring data across 124 sites in the Seto Inland Sea, Japan. BSD was obtained in 67% and 63% of the sites in the spring and summer, respectively, and 37% and 19% of the sites in autumn and winter. High turbulence during the autumn and winter caused unstable background light attenuation, making it difficult to determine an accurate BSD in many sites. BSD was positively correlated with salinity and water depth and could be estimated from these two parameters. Low BSD was observed near the shore and at shallow sites. Measured Secchi depths over 30% smaller than BSD were observed from 20% to 48% of the sites in 1981–1990, those observations were reduced to 4% to 9% of the sites in the past 10 years (2005–2014), indicating that the impact of eutrophication on light transmittance is not severe at present. Consequently, further reduction of anthropogenic nutrient loads will have a limited effect on Secchi depth in many areas. Moreover, we show the existence of a greater discrepancy between measured Secchi depth and BSD in warm seasons than in cold seasons. Similarly, the difference between measured Secchi depth and BSD was higher in geographically enclosed sub-basins than in other sub-basins.

Finding the harvesting frequency to maximize nutrient removal in a constructed wetland dominated by submerged aquatic plants

Water quality is still poor in many freshwater ecosystems around the world as a result of anthropogenic nutrient loading. Constructed wetlands can be used to remove excess nutrients. In these wetlands, helophytes or free floating aquatic plants are traditionally used to absorb the nutrients. The nutrients are subsequently exported upon harvesting of the plants. However, rooted submerged plants may be more effective to extract nutrients from moderately eutrophicated ecosystems than helophytes or floating species.Here, we tested how the frequency of harvesting affected submerged biomass production, biomass nutrient content and the resulting amount of nutrients removed, as well as the vegetation composition and structure. Two Myriophyllum spicatum dominated shallow ponds, with moderately low surface water nutrient loading (∼5.6mgN.m−2.d−1 and ∼1.32mg P.m−2.d−1) were used. Each pond was subjected to four harvesting treatments: mowing 1x, 2x, 3x or 5x between May and September 2015.Harvesting 2x or 3x removed most biomass and nutrients, while mowing either 5x or only once at the end of the growing season removed the lowest amount of nutrients from the system. Furthermore, the dominance of M. spicatum in the vegetation was best maintained in plots mown 2x, while its cover declined in plots mown more frequently, resulting in an increase of charophyte abundance.We conclude that harvesting at an intermediate frequency is best when aiming to remove the maximum amount of nutrients under a moderately low nutrient loading. Harvesting more frequently may be a suitable management method to reduce dominance of M. spicatum in situations where it causes nuisance problems due to massive growth.

Eutrophication decreases salt marsh resilience through proliferation of algal mats

Globally, many estuaries are affected by nutrient loading from human land uses in the surrounding watersheds. One consequence of increased nutrient levels is proliferation of opportunistic macroalgae. We sought to understand spatial and temporal dynamics of ephemeral macroalgal mats and to examine their effects on salt marsh in a eutrophic estuary in central California. A time series analysis spanning 80years revealed that algal wrack has increased exponentially in frequency on the salt marsh, and was highly correlated with nutrient concentrations in the estuary, which have increased along with fertilizer use. Analysis of sediment δ15N showed a dramatic increase in nutrient loads attributable to agricultural fertilizer over the past 50years. We monitored 15 salt marsh plots along the bank edge and detected a negative relationship between algal wrack cover and salt marsh cover, flowering, and canopy height. Moreover, algal wrack led to retreat of vegetation from the bank edge, and increased bank erosion. We also experimentally added algal wrack to salt marsh edge plots. Algal addition decreased salt marsh cover, flowering, and canopy height, and increased retreat rate. By integrating time series analyses, isotope data, algal and marsh monitoring and manipulative experiments, we have identified robust linkages between increased anthropogenic nutrient loading, increased algal wrack cover, reduction in marsh resilience and conversion of marsh habitat to mudflat through bank erosion. Decreasing nutrient inputs to eutrophic estuaries is thus essential for conservation and restoration of salt marshes and enhancing their resilience in the face of sea level rise.

Algal richness and life‐history strategies are influenced by hydrology and phosphorus in two major subtropical wetlands

Summary We explored controls of algal taxon richness (hereafter richness) in complex and hydrologically dynamic flood‐pulsed wetlands by comparing results from independent studies in two globally important subtropical wetlands: the Okavango Delta (Botswana) and the Florida Everglades (U.S.A.). In both wetlands, the flood pulse hydrology is regulated by distinct wet and dry seasons, and creates floodplain landscapes with heterogeneous habitats; algal growth is limited by phosphorus (P); and water uses threaten ecosystem function. To inform future comparisons of algal richness and distribution patterns, we assessed the role of hydrology and P as key controls of richness, and identified indicator taxa of desiccation disturbance and P scarcity in these wetlands under increasing hydrological, nutrient, and habitat changes. We used the intermediate disturbance hypothesis, and the species‐energy theory to explain algal richness patterns, and the competitive, stress‐tolerant, ruderal (CSR) framework to classify indicator taxa. We collected algal samples, environmental data and information expected to influence community structure (water depth, relative depth change, P concentrations, hydroperiod and habitat type) over several years at sites representing a broad range of environmental characteristics. To account for sample size differences, we estimated algal richness by determining the asymptote of taxon accumulation curves. Using multiple regression analysis, we assessed if and how water depth, depth change, P, hydroperiod, and habitat type influence richness within each wetland. We then compared the strength of the relationships between these controlling features and richness between wetlands. Using indicator species analysis on relative abundance data, we classified C, S and R indicator taxa associated with shorter/longer hydroperiod, and lower/higher P concentrations. In either wetland, we did not observe the negative unimodal relationship between site‐specific richness and water depth change that was expected following the intermediate disturbance hypothesis. It is possible that this relationship exists at more highly resolved temporal scales than the semi‐annual to annual scales hypothesised here. However, as nutrient flows and algal habitats depend on these wetlands' flood pulse, maintaining the Okavango's natural pulse, and increasing freshwater flow in the Everglades would help protect these wetlands' algal diversity. Chlorophyta richness (Okavango), and total, Bacillariophyta, Chlorophyta and cyanobacteria richness (Everglades) increased with higher P concentrations, as per species‐energy theory. In the Okavango, we classified 6 C and 49 R indicator taxa (e.g. many planktonic Chlorophyta), and in the Everglades, 15 C, 1 S and 9 R taxa (e.g. benthic Bacillariophyta and planktonic/benthic Chlorophyta), and one stress‐ and disturbance‐tolerant cyanobacterium species. Our results offer baseline information for future comparisons of richness, and abundance of C, S and R indicator taxa in subtropical wetlands; this can be used to quantify how algal communities may respond to potential changes in hydrology and P due to water diversion, anthropogenic nutrient loads, and climate change. Examining microhabitat heterogeneity, nitrogen and light availability, and grazing pressure in such wetlands would further illuminate patch‐scale controls of richness and life‐history strategy distribution in algal communities.

Suspended-Sediment Impacts on Light-Limited Productivity in the Delaware Estuary

The Delaware Estuary has a history of high anthropogenic nutrient loadings but has been classified as a high-nutrient, low-growth system due to persistent light limitation caused by turbidity. While the biogeochemical implications of light limitation in turbid estuaries have been well-studied, there has been minimal effort focused on the connectivity between hydrodynamics, sediment dynamics, and light limitation. Our understanding of sediment dynamics in the Delaware Estuary has advanced significantly in the last decade, and this study describes the impact of spatiotemporal variability of the estuarine turbidity maximum (ETM) on light-limited productivity. This analysis uses data from eight along-estuary cruises from March, June, September, and December 2010 and 2011 to evaluate the impact of the turbidity maximum on production. Whereas the movement of the ETM is controlled primarily by river discharge, the structure of the ETM is modulated by stratification, which varies with both river discharge and spring-neap conditions. We observe that the ETM’s location and structure control spatial patterns of light availability. To evaluate the relative contributions of river discharge and spring-neap variability to the location of phytoplankton blooms, we develop an idealized two-dimensional Regional Ocean Modeling System (ROMS) numerical model. We conclude that high river flows and neap tides can drive stratification that is strong enough to prevent sediment from being resuspended into the surface layer, thus providing light conditions favorable for primary production. This study sheds light on the role of stratification in controlling sediment resuspension and promoting production, highlighting the potential limitations of biogeochemical models that neglect sediment processes.

Stable Isotopes Reveal Nitrogen Loading to Lake Tanganyika from Remote Shoreline Villages.

Access to safe water is an ongoing challenge in rural areas in Tanzania where communities often lack access to improved sanitation. Methods to detect contamination of surface water bodies, such as monitoring nutrient concentrations and bacterial counts, are time consuming and results can be highly variable in space and time. On the northeast shore of Lake Tanganyika, Tanzania, the low population density coupled with the high potential for dilution in the lake necessitates the development of a sensitive method for detecting contamination in order to avoid human health concerns. We investigated the potential use of nitrogen and carbon stable isotopes of snail tissues to detect anthropogenic nutrient loading along the northeast shore of Lake Tanganyika. δ15N of snails was positively related to human population size in the nearest village, but only for villages with >4000 inhabitants. The areal footprint of villages within their watershed was also significantly correlated with snail δ15N, while agricultural land use and natural vegetation were not. Dissolved nutrient concentrations were not significantly different between village and reference sites. Our results indicate that nitrogen isotopes provide a sensitive index of local nutrient loading that can be used to monitor contamination of oligotrophic aquatic environments with low surrounding population densities.

Are seagrass beds indicators of anthropogenic nutrient stress in the rocky intertidal?

It is well established that anthropogenic nutrient inputs harm estuarine seagrasses, but the influence of nutrients in rocky intertidal ecosystems is less clear. In this study, we investigated the effect of anthropogenic nutrient loading on Phyllospadix spp., a rocky intertidal seagrass, at local and regional scales. At sites along California, Washington, and Oregon, we demonstrated a significant, negative correlation of urban development and Phyllospadix bed thickness. These results were echoed locally along an urban gradient on the central California coast, where Phyllospadix shoot δ15N was negatively associated with Phyllospadix bed thickness, and experimentally, where nutrient additions in mesocosms reduced Phyllospadix shoot formation and increased epiphytic cover on Phyllospadix shoots. These findings provide evidence that coastal development can threaten rocky intertidal seagrasses through increased epiphytism. Considering that seagrasses provide vital ecosystem services, mitigating eutrophication and other factors associated with development in the rocky intertidal coastal zone should be a management priority.

Spatial and temporal distributions of Secchi depths and chlorophyll a concentrations in the Suo Nada of the Seto Inland Sea, Japan, exposed to anthropogenic nutrient loading

Thirty years of monitoring data were used to elucidate the spatial and temporal distributions of Secchi depths in the Suo Nada (Suo Sea) and to evaluate how chlorophyll a concentration and reductions of nutrient loading from the watershed affected those distributions. Secchi depths throughout the Suo Nada were positively correlated with water depths. The spatial and temporal variations of Secchi depths could be explained by variations of phytoplankton biomass in areas where the water depth exceeded 20m, but in areas shallower than 10m, other factors affecting light attenuation beside phytoplankton, which include suspended particulate matter and chromophoric dissolved organic matter, obscured relationships between phytoplankton biomass and Secchi depths. Phosphorus limited phytoplankton biomass in the Suo Nada. The main source of allochthonous phosphorus from the 1980s to the 1990s was the watershed. Because of significant reductions of nutrient loading from the watershed, the Pacific Ocean will most likely be the principal source of allochthonous phosphorus after around 2000, except in areas shallower than 10m.

Connectivity between estuaries influences nutrient transport, cycling and water quality

Water quality in coastal and estuarine systems is dictated by nutrient supply and circulation. However, convoluted coastlines, intermittent river discharge, complex currents, and variable nutrient inputs and transformations, confound interpretation of sparse water quality observations. In high use regions with competing human activities, this system complexity precludes identification of factors causing poor water quality, which can lead to conflict between users. High resolution 3D hydrodynamic and biogeochemical models can simulate the dynamics of a region and demonstrate nutrient pathways in convoluted systems. Here, we use a 3D coupled hydrodynamic, sediment and biogeochemical model to investigate the fine-scale (<100m; <1day resolution) nutrient dynamics of temperate coastal waters in southeast Tasmania, Australia. The fate and transport of anthropogenic nutrients between the Huon estuary, the D'Entrecasteaux Channel and the Derwent estuary is of concern and we use the model to explore the regional contribution and connectivity of marine, catchment and anthropogenic nutrient loads. Model skill in the year 2009 is assessed against water quality observations throughout the region and nutrient budgets are evaluated for each sub-region. Results show seasonal variation in estuarine flow with persistent anthropogenic nutrient sources augmenting phytoplankton production, and reducing water quality in summer months. Nutrient-enriched Huon Estuary and D'Entrecasteaux Channel waters are flushed north to the mouth of the Derwent Estuary and then south into the open ocean, with greatest flux in winter. This demonstrates that water quality in the Derwent is not compromised by nutrient influx from the adjacent waters. By using fine-scale 3D biogeochemical models, such as demonstrated here, we can quantify nutrient transport pathways in coastal waters and resolve conflicts between users to support integrated marine management.

Rapid nitrification of wastewater ammonium near coastal ocean outfalls, Southern California, USA

In the southern California Bight (SCB), there has been a longstanding hypothesis that anthropogenic nutrient loading is insignificant compared to the nutrient loading from upwelling. However, recent studies have demonstrated that, in the nearshore environment, nitrogen (N) flux from wastewater effluent is equivalent to the N flux from upwelling. The composition of the N pool and N:P ratios of wastewater and upwelled water are very different and the environmental effects of wastewater discharges on coastal systems are not well characterized. Capitalizing on routine maintenance of the Orange County Sanitation District’s ocean outfall, wherein a wastewater point source was “turned off” in one area and “turned on” in another for 23 days, we were able to document changes in coastal N cycling, specifically nitrification, related to wastewater effluent. A “hotspot” of ammonium (NH4+) and nitrite (NO2−) occurred over the ocean outfall under normal operations and nitrification rates were significantly higher offshore when the deeper outfall pipe was operating. These rates were sufficiently high to transform all effluent NH4+ to nitrate (NO3−). The dual isotopic composition of dissolved NO3− (δ15NNO3 and δ18ONO3) indicated that N-assimilation and denitrification were low relative to nitrification, consistent with the relatively low chlorophyll and high dissolved oxygen levels in the region during the study. The isotopic composition of suspended particulate organic matter (POM) recorded low δ15NPN and δ13CPN values around the outfall under normal operations suggesting the incorporation of “nitrified” NO3− and wastewater dissolved organic carbon into POM. Our results demonstrate the critical role of nitrification in nitrogen cycling in the nearshore environment of urban oceans.

Trophic cascades on the edge: fostering seagrass resilience via a novel pathway.

Despite widespread degradation, some coastal ecosystems display remarkable resilience. For seagrasses, a century-old paradigm has implicated macroalgal blooms stimulated by anthropogenic nutrient, loading as a primary driver of seagrass decline, yet relatively little attention has been given to drivers of seagrass resilience. In Elkhorn Slough, CA, an estuarine system characterized by extreme anthropogenic nutrient loading and macroalgal (Ulva spp.) blooms, seagrass (Zostera marina) beds have recovered concurrent with colonization of the estuary by top predators, sea otters (Enhydra lutris). Here, we follow up on the results of a previous experiment at the seagrass interior, showing how sea otters can generate a trophic cascade that promotes seagrass. We conducted an experiment and constructed structural equation models to determine how sea otters, through a trophic cascade, might affect the edge of seagrass beds where expansion occurs. We found that at the edge, sea otters promoted both seagrass and ephemeral macroalgae, with the latter contributing beneficial grazers to the seagrass. The surprising results that sea otters promote two potentially competing vegetation types, and a grazer assemblage at their boundary provides a mechanism by which seagrasses can expand in eutrophic environments, and contributes to a growing body of literature demonstrating that ephemeral macroalgae are not always negatively associated with seagrass. Our results highlight the potential for top predator recovery to enhance ecosystem resilience to anthropogenic alterations through several cascading mechanisms.

Bases limnologicas para la gestion de los lagos urbanos de Concepcion, Chile

Morphological, physical and chemical characteristics of urban lakes of Conception were studied. The results indicate that these lakes are in an advanced process of eutrophication caused by excessive anthropogenic nutrient loads (mainly phosphorus and nitrogen). Furthermore, high concentrations of faecal and total coliforms in the water column and heavy metals in the sediments of these lakes were recorded. The principal component analyses showed that total phosphorus, ammonia, pH, conductivity, transparency and chlorophylla, are the variables that best explain the water conditions of all the lakes studied. It is recommended to orientate the lake management actions to decrease total phosphorus. Considering the particular situation of each lake, a series of complementary measures for reversing the current situation should be applied. This study provides new limnological information for the decision-making process focussed on the recovery of urban lakes of the City of Concepcion.

Functional Responses of Salt Marsh Microbial Communities to Long-Term Nutrient Enrichment.

ABSTRACTEnvironmental nutrient enrichment from human agricultural and waste runoff could cause changes to microbial communities that allow them to capitalize on newly available resources. Currently, the response of microbial communities to nutrient enrichment remains poorly understood, and, while some studies have shown no clear changes in community composition in response to heavy nutrient loading, others targeting specific genes have demonstrated clear impacts. In this study, we compared functional metagenomic profiles from sediment samples taken along two salt marsh creeks, one of which was exposed for more than 40 years to treated sewage effluent at its head. We identified strong and consistent increases in the relative abundance of microbial genes related to each of the biochemical steps in the denitrification pathway at enriched sites. Despite fine-scale local increases in the abundance of denitrification-related genes, the overall community structures based on broadly defined functional groups and taxonomic annotations were similar and varied with other environmental factors, such as salinity, which were common to both creeks. Homology-based taxonomic assignments of nitrous oxide reductase sequences in our data show that increases are spread over a broad taxonomic range, thus limiting detection from taxonomic data alone. Together, these results illustrate a functionally targeted yet taxonomically broad response of microbial communities to anthropogenic nutrient loading, indicating some resolution to the apparently conflicting results of existing studies on the impacts of nutrient loading in sediment communities.IMPORTANCE In this study, we used environmental metagenomics to assess the response of microbial communities in estuarine sediments to long-term, nutrient-rich sewage effluent exposure. Unlike previous studies, which have mainly characterized communities based on taxonomic data or primer-based amplification of specific target genes, our whole-genome metagenomics approach allowed an unbiased assessment of the abundance of denitrification-related genes across the entire community. We identified strong and consistent increases in the relative abundance of gene sequences related to denitrification pathways across a broad phylogenetic range at sites exposed to long-term nutrient addition. While further work is needed to determine the consequences of these community responses in regulating environmental nutrient cycles, the increased abundance of bacteria harboring denitrification genes suggests that such processes may be locally upregulated. In addition, our results illustrate how whole-genome metagenomics combined with targeted hypothesis testing can reveal fine-scale responses of microbial communities to environmental disturbance.

Nitrogen limitation of algal biofilms in coastal wetlands of Lakes Michigan and Huron

Coastal wetlands of the Laurentian Great Lakes are critical habitats for many plants and animals of ecological and economic importance and are particularly susceptible to nutrient runoff from the landscape given their proximal location to inflows. We used nutrient diffusing substrata (NDS) to determine whether N, P, or both nutrients limited algal biomass accrual in 54 wetlands of Lakes Michigan and Huron. N was the most common limiting nutrient. Forty-three percent of NDS assays exhibited N limitation, 18% exhibited N+P colimitation, 3% exhibited P limitation, and 36% exhibited neither N nor P limitation. The magnitude of nutrient limitation was negatively correlated with water-column nutrients (dissolved and total N and P) and surrounding agricultural and developed land cover, results suggesting that anthropogenic nutrient loading partially relieved nutrient limitation in many locations. We also analyzed benthic algal assemblages to assess whether N2-fixing taxa were associated with N-limited wetlands. N2-fixing algae were common in our study wetlands, and their abundance was related to nutrient conditions. For example, the percentage of N2-fixing diatoms was significantly higher in N-limited wetlands, positively correlated with the magnitude of N limitation, and negatively correlated with dissolved N concentrations. Our results demonstrate that anthropogenic N loading, which receives considerably less management attention than P in the Great Lakes, can affect coastal wetland algal communities. Benthic algae is an important energy source for much of the food web in Great Lakes coastal wetlands, so effects associated with N loading may have broad implications.

Metabolic and physiological changes in Prymnesium parvum when grown under, and grazing on prey of, variable nitrogen:phosphorus stoichiometry

Mixotrophy is found in almost all classes of phytoplankton in a wide range of aquatic habitats ranging from oligotrophic to eutrophic marine and freshwater systems. Few studies have addressed how the nutritional status of the predator and/or the prey affects mixotrophic metabolism despite the realization that mixotrophy is important ecologically. Laboratory experiments were conducted to examine changes in growth rates and physiological states of the toxic haptophyte Prymnesium parvum when fed Rhodomonas salina of varying nutritional status. Haemolytic activity of P. parvum and prey mortality of R. salina were also measured. P. parvum cultures grown to be comparatively low in nitrogen (low-N), phosphorus (low-P) or low in both nutrients (low-NP) were mixed with low-NP, low-N, and low-P R. salina in all possible combinations, i.e., a 3×3 factorial design. N deficiency was obtained in the low-N cultures, while true P deficiency may not have been obtained in the low-P cultures. Mortality rates of R. salina (both due to ingestion and/or cell rupture as a function of grazing or toxic effects) were higher when R. salina cells were low-P, N-rich, regardless of the nutritional state of P. parvum. Mortality rates were, however, directly related to the initial prey:predator cell ratios. On the other hand, growth of the predator was a function of nutritional status and a significant positive correlation was observed between growth rates of P. parvum and cell-specific depletion rates of N, whereas no such relationship was found between P. parvum growth rates and depletion rates of P. In addition, the greatest changes in chlorophyll content and stoichiometric ratios of P. parvum were observed in high N:P conditions. Therefore, P. parvum may show enhanced success under conditions of higher inorganic N:P, which are likely favored in the future due to increases in eutrophication and altered nutrient stoichiometry driven by anthropogenic nutrient loads that are increasingly enriched in N relative to P.

Allelopathic potential and ecotoxicity evaluation of gallic and nonanoic acids to prevent cyanobacterial growth in lentic systems: A preliminary mesocosm study

The increase in anthropogenic nutrient loading affecting many freshwater ecosystems combined with global warming may lead to cyanobacterial blooms on an increasingly frequent basis. Among the various physicochemical and biological methods which have been proposed to rapidly control blue-green algae growth, the use of plant-derived substances such as allelochemicals has gained great interest as an environment-friendly approach. The primary aim of this work was to evaluate the efficiency of gallic and nonanoic acid application to preemptively inhibit cyanobacterial growth in lentic hydrosystems. In order to address the process feasibility under realistic exposure scenarios, thirteen outdoor freshwater mesocosms (unit volume: 3m3) were designed, each containing phytoplankton (including local blue-green algae species) and various non-target organisms from higher trophic levels (Physa, Lymnaea, Gammarus, and Scardinius erythrophthalmus). After an 8-week mesocosm stabilization period, a full factorial design based on the presence/absence of gallic acid (GA) and nonanoic acid (NA) (including a control group) was implemented into the exposure tanks. Regular monitoring of major phytoplankton taxa was conducted during a 28-day experiment using an on-line fluorometer. The main results suggested that gallic acid was more efficient than nonanoic acid at limiting cyanobacterial growth at concentrations as low as 1mgL−1. Successive gallic acid applications (at 1, 2 and 4mgL−1) at the early stages of cyanobacterial growth did not allow the complete elimination of blue-green algae from the mesocosms. However, the specificity of the allelopathic effect of gallic acid towards cyanobacteria was compatible with the maintenance of a primary productivity in the treated tanks as indicated by the photoautotrophic growth of other algal taxa. Finally, no biomarker induction signal could be reported in non-target species. Further gallic acid application trials in lentic systems such as small freshwater ponds may help to design innovative allelopathy-based aquatic ecotechnologies.

A new perspective on coastal hypoxia: The role of saline groundwater

Processes leading to the development of coastal hypoxia are usually attributed to anthropogenic nutrient loading. We present new evidence documenting a natural process capable of producing large-scale water masses undersaturated with respect to dissolved oxygen, independent of human-caused nutrient loading and subsequent nutrient-enhanced productivity. Along the heavily developed coastline of Myrtle Beach, South Carolina, the offshore discharge of saline, anoxic groundwater generates undersaturated bottom water masses which can be advected into nearshore waters. Highly elevated radium isotope activities measured in low-oxygen bottom waters cannot be supported by any known terrestrial source. Rather, discharge of cold, anoxic, high-radium water from offshore carbonate aquifers is the likely source. Density stratification inhibits mixing of these discharged waters during inshore transport. During a low-oxygen event in August 2012, this restricted mixing is likely the mechanism for the observed oxygen deficit in bottom waters. This discovery suggests benthic layers undersaturated with respect to dissolved oxygen may be considerably more prevalent than currently recognized, particularly around the carbonate-platform of the US East Coast.