Effects Of Nutrient Enrichment Research Articles

Heat Stress Promotes Nitrogen Accumulation in Meristems via Apical Blade Erosion in a Brown Macroalga With Intercalary Growth

Heat stress is known to induce photoinhibition, leaf senescence, and nutrient remobilization in terrestrial plants with apical growth; however, its effect on blade erosion and associated-changes in chemical compositions has rarely been studied in marine macroalgae with intercalary growth such as kelp. The present study examined the combined effects of ocean warming (23 and 26 °C), irradiance (30 and 180 µmol m-2 s-1), and nutrient enrichment (enriched and non-enriched) on photosystem II maximum efficiency (Fv/Fm) in the kelp Eisenia bicyclis. It also investigated the effect of ocean warming on the kelp’s relative growth rates based on five morphological parameters and three chemical compositions (carbon, nitrogen, and phlorotannnins). A warming effect on photoinhibition (i.e., decline in Fv/Fm) was only detected under the higher irradiance combined with nutrient-enrichment condition. Under this condition, elevated temperature decreased relative growth rates to negative values, indicating occurrence of apical blade erosion. Temperature elevation also caused increases in nitrogen and phlorotannin contents within the whole body, but not carbon content. Moreover, nitrogen content in the meristems at 26oC was higher than that at 23oC, although such a difference was not observed with phlorotannin content. These results suggested that heat-induced apical blade erosion promoted nitrogen accumulation in the meristems, located in the lower part of the blade, in E. bicyclis.

Water Column Turbidity Not Sediment Nutrient Enrichment Moderates Microphytobenthic Primary Production

Soft sediment intertidal habitats are under intense anthropogenic pressure resulting from increased land derived sediment and nutrient delivery. Long term, this can cause high water column turbidity and nutrient enrichment of sediment porewaters, which has cascading effects on coastal ecosystem functionality. However, how these stressors may interact and influence benthic productivity over alternating periods of submergence and emergence is largely unknown. This study investigates the effects of sediment nutrient enrichment (at three levels for 20 months) on benthic primary production at six sites in four New Zealand estuaries that spanned a gradient in water column turbidity. While nutrient enrichment had no detectable effect on microphytobenthic primary production, water column turbidity had a significant influence, explaining up to 40% of variability during tidal submergence, followed by temperature and sediment characteristics. In addition, negative net primary production (NPP) estimates and therefore net heterotrophy for the most turbid estuaries during tidal submergence resulted in an increased reliance on production during emerged periods, where NPP was positive across all sites. This study highlights the prominent role of water column turbidity over nutrient enrichment in moderating microphytobenthic production, and the increasing importance of emerged periods to maintain the health and functioning of coastal habitats.

A shift in the pool of retained microphytobenthos nitrogen under enhanced nutrient availability

Sediment microbial communities are an important sink for both organic and inorganic nitrogen (N), with microphytobenthos (MPB) biomass making the largest contribution to short-term N-assimilation and retention. Coastal waters are increasingly subject to anthropogenic nutrient enrichment, but the effect of nutrient enrichment on microbial assimilation, processing, and fate of MPB-derived N (MPB-N) remains poorly characterised. In this study, an MPB-dominated microbial community was labeled in situ with a pulse of 15NH4+-N. Laboratory core incubations of this labeled sediment under increasing nutrient concentrations (NH4+ and PO43−: ambient, 2 × ambient, 5 × ambient, and 10 × ambient) were used to investigate changes in the processing and flux pathways of the 15N-labeled MPB-N across 10.5 d under nutrient enrichment. Short-term retention of MPB-N by MPB was stimulated by nutrient addition, with higher 15N in MPB in the nutrient amended treatments (71–93%) than in the ambient treatment (38%) at 0.5 d After 10.5 d, the nutrient amended treatments had increased turnover of MPB-N out of MPB biomass into an uncharacterised pool of sediment ON (45–75%). Increased turnover of MPB-N likely resulted from decreased recycling of MPB-N between MPB and heterotrophic bacteria as inorganic nutrients were preferentially used as an N source and remineralisation of sediment ON decreased. Decreased breakdown of sediment ON reduced the efflux of MPB-N via DON in the amended (3.9–5.2%) versus the ambient treatment (10.9%). Exports of MPB-N to the water column were relatively small, accounting for a maximum of 14% of 15N exported from the sediment, and were predominantly exported DON and N2 (denitrification). Overall, there was considerable retention of MPB-N over 10.5 d, but increased nutrient loading shifted N from MPB biomass into other sediment ON.

Diversity-dependent soil acidification under nitrogen enrichment constrains biomass productivity.

In most plant communities, the net effect of nitrogen enrichment is an increase in plant productivity. However, nitrogen enrichment also has been shown to decrease species richness and to acidify soils, each of which may diminish the long-term impact of nutrient enrichment on productivity. Here we use a long-term (20year) grassland plant diversity by nitrogen enrichment experiment in Minnesota, United States (a subexperiment within the BioCON experiment) to quantify the net impacts of nitrogen enrichment on productivity, including its potential indirect effects on productivity via changes in species richness and soil pH over an experimental diversity gradient. Overall, we found that nitrogen enrichment led to an immediate positive increment in productivity, but that this effect became nonsignificant over later years of the experiment, with the difference in productivity between fertilized and unfertilized plots decreasing in proportion to nitrogen addition-dependent declines in soil pH and losses of plant diversity. The net effect of nitrogen enrichment on productivity could have been 14.5% more on average over 20years in monocultures if not for nitrogen-induced decreases in pH and about 28.5% more on average over 20years in 16 species communities if not for nitrogen-induced species richness losses. Together, these results suggest that the positive effects of nutrient enrichment on biomass production can diminish in their magnitude over time, especially because of soil acidification in low diversity communities and especially because of plant diversity loss in initially high diversity communities.

Effects of experimental warming and nutrient enrichment on wetland communities at the Arctic’s edge

Global warming-related changes to freshwater ecosystems in Arctic and Subarctic regions have been magnified by nutrient input from increasing waterfowl populations. To gain insight into how these changes might affect ecosystem function, we conducted a mesocosm experiment in the Subarctic by enriching N and P (1 ×, 10 ×, and 20 × treatments) and increasing mean water temperatures ≤ 3°C. We measured responses of two species of larval amphibians, periphyton, and phytoplankton. Wood frog (Rana sylvatica) larvae developed quicker (odds ratio [OR] for 1°C increase = 0.903, 95% CI 0.892–0.912) and were more likely to metamorphose (OR 1.076, 95% CI 0.022–14.73) in warmer waters. Boreal chorus frogs (Pseudacris maculata) also developed quicker with warmer temperatures (OR 0.880, 95% CI 0.860–0.900), despite a non-significant trend toward reduced survival (OR 0.853, 95% CI 0.696–1.039). Periphyton and phytoplankton concentrations increased with nutrient additions, as did size of wood frog metamorphs. Periphyton and phytoplankton did not vary with temperature, but periphyton was limited by tadpole abundance. Our results highlight the potential for non-linear responses to ecosystem change, with species-specific consumer and ecosystem responses that depend on the magnitude of changes.

Effect of Nutrient Enrichment and Turbidity on Interactions Between Microphytobenthos and a Key Bivalve: Implications for Higher Trophic Levels

© Copyright © 2020 Hope, Hewitt, Pilditch, Savage and Thrush. Benthic diatoms are a high-quality food resource providing essential fatty acids to benthic grazers. Different stressors may alter the proportion of diatoms and other microalgae and thus can affect the quality as well as quantity of food available to benthic consumers. Microphytobenthos (MPB) lipid biomarkers were assessed in a field experiment to elucidate changes to the biosynthesis of fatty acids (FA) under nitrogen (N) enrichment (three levels) at eight intertidal sites that spanned a turbidity gradient. Influences on the flow of carbon and energy were determined using FA biomarkers of a functionally important deposit-feeding tellinid bivalve (Macomona liliana). Site-specific effects of N enrichment were detected in MPB quantity and quality measurements. Enrichment generally increased MPB biomass (chl a) across all sites, while the proportion of diatom associated fatty acid biomarkers was more variable at some sites. Analysis of sediment FA biomarkers and environmental variables suggested that changes to the microbial community composition and quality were related to water clarity and mud content of the bed. The ability of the MPB to utilize the increased nitrogen, as indicated by the resource use efficiency index, was also important. Despite the increase in MPB biomass, lipid reserves in the tissue of M. liliana, a primary consumer of MPB, were reduced (by up to 6 orders of magnitude) in medium and high N addition plots compared to control plots. Further, the nutritional quality of the bivalves to higher trophic levels [indicated by a lower ratio of essential FAs (ω3:ω6)] was reduced in high treatment plots compared to control plots suggesting the bivalves were adversely affected by nutrient enrichment but not due to a reduction in food availability. This study suggests anthropogenic nutrient enrichment and turbidity may indirectly alter the structure and function of the benthic food web, in terms of carbon flow and ecosystem productivity. This may indirectly change the interactions between MPB and key bivalves as suspended sediment concentrations and nutrient enrichment continue to increase globally. This has implications for various ecosystem functions that are mediated by these interactions, such as nutrient cycling as well as primary and secondary production.

Reducing nutrient availability and enhancing biotic resistance limits settlement and growth of the invasive Australian swamp stonecrop (Crassula helmsii)

The invasive Australian swamp stonecrop, Crassula helmsii, is a perennial amphibious herb originating from Australia and New Zealand. In freshwater wetlands of North-western Europe, this alien plant species is invasive due to its efficient colonization of empty niches. The establishment of dense C. helmsii growth is threatening native biodiversity and functioning of freshwater ecosystems, especially oligotrophic wetlands with high disturbance and nutrient enrichments. As the effects of these potential drivers of ecosystem degradation are generally difficult to determine in the field, we tested the competitive strength of C. helmsii in a greenhouse experiment with two native competitor species of the same habitat type, Pilularia globulifera and Littorella uniflora. Sods dominated by either of the native species, as well as bare soils, were collected from the field and manually infested with propagules of C. helmsii. Settlement and growth of C. helmsii was assessed after five weeks. In addition, the effect of nutrient enrichment by water bird feces on competition was studied by adding waterfowl droppings. C. helmsii was able to settle successfully in all treatments, but P. globulifera and L. uniflora dominance reduced settlement success and growth of C. helmsii. On vegetated sods, the addition of waterfowl droppings had a low effect on the performance of C. helmsii, however, this treatment significantly increased biomass production on bare soils with low nutrient availability. We conclude that both absence of native competitors and eutrophication, including guanotrophication by waterfowl, explain the establishment success and invasiveness of C. helmsii. Given the fact that eradication of C. helmsii is very challenging, our results imply that management should focus on a combination of increasing local species densities and abating eutrophication. This will strongly limit the window of opportunity for invasion of C. helmsii and enhance resistance by native plant communities.

Indicators of nutrient enrichment in coastal ecosystems of the northern Mexican Caribbean

We measured dissolved inorganic nitrogen (DIN = NH4+ + NOx [= NO3− + NO2−]), soluble reactive phosphorous (SRP), and ecological indicators (chlorophyll-a concentration, coverage of submerged aquatic vegetation, and %N, C:N and δ15N values in the seagrass Thalassia testudinum) to evaluate the effects of nutrient enrichment at three shallow coastal sites on the northern part of the Mexican Caribbean (Puerto Morelos, Puerto Juárez, and Isla Blanca) in two contrasting (cold-front or dry) seasons of the year. High nutrient concentrations prevailed in both seasons but were significantly higher during the cold-front season, probably as a result of sediment resuspension and groundwater discharges. The highest δ15N values were recorded in Puerto Juárez in both seasons (cold-front, 7.58‰; dry, 7.35‰). The highest macroalgae coverage and NH4+ concentration were also recorded in this site. These results, together with the high correlation between δ15N and NH4+ (r = 0.94) and DIN (r = 0.88) concentrations, suggest wastewater contamination in Puerto Juárez due to its proximity to the city of Cancún. In contrast, the lowest mean δ15N (−1.35‰) and nutrient enrichment effects (i.e., highest coverage of T. testudinum and lowest macroalgae coverage) were observed in Isla Blanca, a site with no urban settlements. The low δ15N values recorded along the Puerto Morelos coastal transect suggest that a significant fraction of the nitrogen assimilated by T. testudinum may come from either exudates or the decomposition of drifting Sargassum stranded in the coast, as the δ15N values recorded in Sargassum (−0.64‰) were similar to those found in the seagrass (−0.83‰). The correlation between the DIN:SRP ratio and chlorophyll-a concentration in the study area may indicate N-limitation of phytoplankton growth, despite the high DIN concentrations. Beyond the differences observed between sites – which may indicate site-specific, long-term nutrient inputs – the indicators examined evidence environmental deterioration of the study area, likely due to a shift in the nutrient regime (away from oligotrophic), particularly in Puerto Juárez, the site with the heaviest load of urban and tourism activities.

Deposition of shells modify nutrient fluxes in marine sediments: effects of nutrient enrichment and mitigation by bioturbation below mussel farms

Farming of extractive species such as filter feeding bivalves has been proposed as a potential method to mitigate impacts of eutrophication in marine environments. For such efforts to be sustainable, potential negative effects from mussel farms, such as accumulation of biodeposits in sediment below them, need to be considered and addressed. Benthic burrowing macrofauna strongly influence biogeochemical processes in soft bottom marine habitats by sediment reworking and irrigation and, thus, have the potential to mitigate some of the negative impacts. However, not all biodeposits are organic matter; shells that accumulate on and in the sediment below mussel farms also have the potential to influence processes in the sediment, the activity of bioturbators and the fluxes across the sediment-water interface. In this study, we evaluated the mitigation potential of the bioturbating polychaeteHediste diversicolorin sediments enriched with mussel waste material and the relative impact of mussel shells within the sediment matrix. The polychaetes generally increased fluxes and sediment oxygen uptake. With an observed tendency of increased fluxes of nutrients in sediments containing shells compared to sediments without, the results indicate that the accumulation of shell has a potential to further increase the mitigative effect of the polychaetes by influencing the solute fluxes across the sediment-water interface.

A healthy trophic structure underlies the resistance of pristine seagrass beds to nutrient enrichment

AbstractWhether pristine ecosystems with intact trophic structures are more resilient to anthropogenic pressures than ecosystems facing human exploitation is a pressing question to ecological theory and management. For coastal vegetated ecosystems such as kelp forests and seagrass meadows, a number of studies recently highlighted that under eutrophic conditions, trophic cascades are particularly important in buffering or reducing negative effects of nutrient enrichment. Yet, it currently remains unclear how nutrient enrichment and trophic downgrading interact in oligotrophic coastal ecosystems with an intact trophic structure. Here, we factorially manipulated nutrient loading and the interactions within a tri‐trophic food chain within a pristine, oligotrophic seagrass ecosystem to investigate how trophic downgrading affects its ability to buffer against eutrophication. Results revealed that nutrient addition stimulates seagrass production, while reducing the growth of benthic microalgae, presumably because the thicker seagrass canopy reduced light availability. Trophic downgrading by excluding predators, however, almost completely negated these nutrient effects, as the release of herbivores from predation strongly enhanced grazing pressure on seagrass. Exclusion of grazers in turn restored seagrass biomass by allowing the nutrient addition treatment to regain its effect, confirming that a tri‐trophic cascade mediated how enhanced nutrient loading affected our system. Our results highlight that a healthy trophic structure is vital for the ability of pristine coastal ecosystems to buffer eutrophication, as trophic downgrading may degrade their ability to absorb enhanced nutrient loading. Hence, our findings emphasize the potential for multiple anthropogenic impacts to interact synergistically in degrading natural ecosystems.

Soil acidification reduces the effects of short-term nutrient enrichment on plant and soil biota and their interactions in grasslands.

Soil nitrogen (N) and phosphorus (P) contents, and soil acidification have greatly increased in grassland ecosystems due to increased industrial and agricultural activities. As major environmental and economic concerns worldwide, nutrient enrichment and soil acidification can lead to substantial changes in the diversity and structure of plant and soil communities. Although the separate effects of N and P enrichment on soil food webs have been assessed across different ecosystems, the combined effects of N and P enrichment on multiple trophic levels in soil food webs have not been studied in semiarid grasslands experiencing soil acidification. Here we conducted a short-term N and P enrichment experiment in non-acidified and acidified soil in a semiarid grassland on the Mongolian Plateau. We found that net primary productivity was not affected by N or P enrichment alone in either non-acidified or acidified soil, but was increased by combined N and P enrichment in both non-acidified and acidified soil. Nutrient enrichment decreased the biomass of most microbial groups in non-acidified soil (the decrease tended to be greatest with combined N and P enrichment) but not in acidified soil, and did not affect most soil nematode variables in non-acidified or acidified soil. Nutrient enrichment also changed plant and microbial community structure in non-acidified but not in acidified soil, and had no effect on nematode community structure in non-acidified or acidified soil. These results indicate that the responses to short-term nutrient enrichment were weaker for higher trophic groups (nematodes) than for lower trophic groups (microorganisms) and primary producers (plants). The findings increase our understanding of the effects of nutrient enrichment on multiple trophic levels of soil food webs, and highlight that soil acidification, as an anthropogenic stressor, reduced the responses of plants and soil food webs to nutrient enrichment and weakened plant-soil interactions.

Impacts of multiple stressors on freshwater biota across spatial scales and ecosystems.

Climate and land-use change drive a suite of stressors that shape ecosystems and interact to yield complex ecological responses (that is, additive, antagonistic and synergistic effects). We know little about the spatial scales relevant for the outcomes of such interactions and little about effect sizes. These knowledge gaps need to be filled to underpin future land management decisions or climate mitigation interventions for protecting and restoring freshwater ecosystems. This study combines data across scales from 33 mesocosm experiments with those from 14 river basins and 22 cross-basin studies in Europe, producing 174 combinations of paired-stressor effects on a biological response variable. Generalized linear models showed that only one of the two stressors had a significant effect in 39% of the analysed cases, 28% of the paired-stressor combinations resulted in additive effects and 33% resulted in interactive (antagonistic, synergistic, opposing or reversal) effects. For lakes, the frequencies of additive and interactive effects were similar for all spatial scales addressed, while for rivers these frequencies increased with scale. Nutrient enrichment was the overriding stressor for lakes, with effects generally exceeding those of secondary stressors. For rivers, the effects of nutrient enrichment were dependent on the specific stressor combination and biological response variable. These results vindicate the traditional focus of lake restoration and management on nutrient stress, while highlighting that river management requires more bespoke management solutions.

Ocean warming combined with nutrient enrichment increases the risk of herbivory during cultivation of the marine macroalga Undaria Pinnatifida

Abstract Recent declines of macroalgal forests due to climate change imply that the aquaculture production of macroalgae may also be negatively affected by ocean warming. It has previously been shown that nutrient enrichment can offset the negative impact of warming on the survival and growth of the cultivated macroalga Undaria pinnatifida, although it can also increase the risk of herbivory by unrecognized mesograzers. Observations of several hundreds or thousands of the isopod Cymodocea japonica around U. pinnatifida cultivation ropes in autumn, prompted the analysis of their size-frequency distribution, relationship between body size and consumption rate, and combined effects of elevated temperature and nutrient enrichment on the consumption rate of the algae by the isopod in the current study. Although the body size of the isopods collected in autumn was smaller than those collected in winter and spring, the consumption rate of 600 small isopods was similar to the rate of a large individual. Additionally, the consumption rate doubled in response to an increase in temperature from 15 to 18°C and nutrient enrichment, and tripled with their combination. These results suggest that ocean warming combined with nutrient enrichment increase the risk of herbivory during U. pinnatifida cultivation, especially during isopod outbreak periods.

Environmental context shapes the long‐term role of nutrients in driving producer community trajectories in a top–down dominated marine ecosystem

Abstract Two predominant anthropogenic impacts on ecosystems, nutrient enrichment and the removal of consumers, are predicted to interact in their effects on producer diversity. Yet, measures of diversity alone may not capture changes occurring in the underlying mechanisms structuring communities. Furthermore, evidence for these interactions in rocky intertidal systems is mixed and may be confounded by variable baseline productivity or short experimental durations that do not capture seasonality, environmental heterogeneity or successional processes. We conducted a 2‐year experiment examining the main and interactive effects of nutrients and herbivores in a low‐productivity environment on the northern shore of Cook Strait near Wellington, New Zealand. We explicitly accounted for the small‐scale spatial environmental heterogeneity characteristic of rocky shores. Rapid and dramatic shifts in diversity metrics and cover of algal communities in the first year were driven by removal of herbivores. Within 2 months of herbivore exclusion, open space decreased from >90% to <15%, regardless of nutrient addition, and both algal diversity and evenness plummeted while species richness rose. In contrast to theoretical predictions, strong interactions between top–down and bottom–up forces on diversity were rare; rather, over time, the addition of nutrients negatively impacted diversity. Further, where herbivores were removed, nutrients inhibited succession beyond ephemeral forms, contrasting with the tolerance model of succession observed under ambient nutrients. The effects of nutrients were only elucidated by accounting for variation in habitat complexity and seasonality, demonstrating that environmental heterogeneity can mask bottom–up processes. Moreover, shifts in relative species abundances revealed that after 10 months, nutrient addition caused increased community turnover regardless of herbivore presence or absence, indicating reduced stability not apparent in simple measures of diversity. Synthesis. Our results suggest that nutrient addition and herbivore reduction exert strong but mostly independent control on community biodiversity, where context dependency in habitat complexity, seasonality and successional stage played key roles in determining community outcomes. These findings demonstrate the importance of longer‐term field studies incorporating environmental context and multiple metrics of diversity to reveal the mechanisms underlying effects of nutrient enrichment and herbivore loss on producer community structure.

Ecosystem-scale nutrient cycling responses to increasing air temperatures vary with lake trophic state

Understanding potential effects of climate warming on biogeochemical cycling in freshwater ecosystems is of pressing importance. Specifically, increasing air and water temperatures could accelerate nutrient cycling in lakes, which has major implications for in-lake nutrient concentrations, water column nutrient stoichiometry, and downstream nutrient export. Lakes may respond differentially to warming based on their current trophic state, although direct comparisons of temperature-driven changes in nutrient cycling between low- and high-nutrient lakes are lacking. Here, we used an open-source coupled hydrodynamic biogeochemical model to simulate ecosystem-scale changes in water column total nitrogen (TN) and total phosphorus (TP) concentrations and TN:TP ratios due to potential incremental changes in air temperature (from +0 °C to +6 °C) in a low-nutrient and a high-nutrient lake. Warming resulted in lower TN and higher TP epilimnetic (surface water) concentrations in both lakes, resulting in reduced molar TN:TP ratios in both lakes. While the high- and low-nutrient lakes had similar magnitude reductions in TN:TP ratio between the +0 °C and +6 °C scenarios (30.3% and 34.6%, respectively), median epilimnetic TN:TP in the low-nutrient lake significantly decreased with as little as 1 °C of warming. Warming also altered net nutrient retention, with decreased downstream export of TN but increased downstream export of TP in both lakes. Our modeling results suggest that low-nutrient lakes may respond to warming at lower levels of temperature increase than high-nutrient lakes, and that climate warming could intensify effects of nutrient enrichment driven by increased N and P loading due to land-use change.

Low growth resilience of subarctic rhodoliths (Lithothamnion glaciale) to coastal eutrophication

Eutrophication is one of the most important drivers of change in coastal marine ecosystems worldwide. Given their slow growth, rhodoliths and the biodiverse communities they support are regarded as non-renewable resources threatened by human activity. Consequences of nutrient enrichment on growth and calcification in crustose coralline algae are equivocal, and even more so in cold-water rhodoliths. We paired a 183 d laboratory mesocosm experiment with a 193 d field experiment on Newfoundland (Canada) rhodoliths (Lithothamnion glaciale) to test the hypothesis that nutrient (nitrate, ammonia, and phosphate) enrichment and biofouling reduce rhodolith growth. Rhodoliths in the laboratory were exposed to 1 of 3 nutrient concentrations (ambient, intermediate, or high) and either of 2 levels of manual cleaning (cleaned or uncleaned) to control biofouling. We exposed rhodoliths in the field to 1 of 2 nutrient concentrations (ambient or elevated). Eutrophication in the laboratory did not affect biofouling; however, manual cleaning reduced biofouling by ~4 times relative to uncleaned rhodoliths. Rhodoliths grew 2 times slower at elevated than ambient nutrient concentrations, and ~27% more in cleaned than uncleaned rhodoliths at all concentrations. Rhodoliths in the field also grew significantly slower under elevated than ambient phosphate concentrations, but only during the first 6 wk, indicating some capacity for long-term recovery. We conclude that despite some growth resilience to low and infrequent increases in nutrient concentrations, subarctic L. glaciale rhodoliths cannot cope with prolonged exposure to modest eutrophication.

Nutrient Status of San Francisco Bay and Its Management Implications

Nutrient enrichment has degraded many of the world’s estuaries by amplifying algal production, leading to hypoxia/anoxia, loss of vascular plants and fish/shellfish habitat, and expansion of harmful blooms (HABs). Policies to protect coastal waters from the effects of nutrient enrichment require information to determine if a water body is impaired by nutrients and if regulatory actions are required. We compiled information to inform these decisions for San Francisco Bay (SFB), an urban estuary where the best path toward nutrient management is not yet clear. Our results show that SFB has high nutrient loadings, primarily from municipal wastewater; there is potential for high algal production, but that production is not fully realized; and SFB is not impaired by hypoxia or recurrent HABs. However, our assessment includes reasons for concern: nitrogen and phosphorus concentrations higher than those in other estuaries impaired by nutrient pollution, chronic presences of multiple algal toxins, a recent increase of primary production, and projected future hydroclimatic conditions that could increase the magnitude and frequency of algal blooms. Policymakers thus face the challenge of determining the appropriate protective policy for SFB. We identify three crucial next steps for meeting this challenge: (1) new research to determine if algal toxins can be reduced through nutrient management, (2) establish management goals as numeric targets, and (3) determine the magnitude of nutrient load reduction required to meet those targets. Our case study illustrates how scientific information can be acquired and communicated to inform policymakers about the status of nutrient pollution, its risks, and strategies for minimizing those risks.

Eutrophication sources, impacts and management: A case study from Abu Dhabi

Human activities and coastal development driven by economic growth in countries bordering the Arabian Gulf have increased marine pollution, including eutrophication. In response to growing concerns, the Environment Agency- Abu Dhabi initiated a long-term monitoring survey (2006 to 2019) to study the marine water quality of Abu Dhabi, including the Mussafah South Channel (MSC). This confined area is of eutrophic nature, under pressure due to receiving approximately 400,000 m3 of treated sewage per day and continuous algal blooms incidents with maximum concentration of 18 x 107 cells l−1. Observations made on physical, chemical, and biological parameters, such as temperature (18 -35 °C), salinity (28 – 70 psu), pH (5 to 9), and dissolved oxygen (0.02 – 13 mg l−1) revealed abnormal conditions and a stratified water column. The nutrient values, particularly nitrate (2.90 – 866.06 Mol l−1) and phosphate (1.68 -98.24 Mol l−1), were many times higher than Abu Dhabi’s reference values (3.55 Mol l−1 and 1.58 Mol l−1, respectively) which confirmed its eutrophic nature. Algal blooms were frequently recorded, and were associated with widespread harmful impacts, including hypoxic events (oxygen levels of 0.02 mg l−1), finfish kills (Nematalosa nasus) and subsequent loss of benthic organisms. The deterioration of water quality in the MSC is probably due to the anthropogenic activities, insufficient treatment of municipal and industrial wastewater discharges. Extensive sediment load may also have exacerbated the situation and contributed to eutrophication and subsequent alteration of the ecosystem. The sources, composition and consequences of nutrient enrichment along with the management actions are also discussed in detail.

New insights into the role of nutrient cycling in food web dynamics

Nutrient cycling is fundamental to ecosystem functioning. Despite recent major advances in the understanding of complex food web dynamics, food web models have so far generally ignored nutrient cycling. However, nutrient cycling is expected to strongly impact food web stability and functioning. To make up for this gap, we built an allometric and size structured food web model including nutrient cycling. By releasing mineral nutrients, recycling increases the availability of limiting resources for primary producers and links each trophic level to the bottom of food webs. We found that nutrient cycling can provide a significant part of the total nutrient supply of the food web, leading to a strong enrichment effect that promotes species persistence in nutrient poor ecosystems but leads to a paradox of enrichment at high nutrient inputs. The presence of recycling loops linking each trophic level to the basal resources weakly affects species biomass temporal variability in the food web. Recycling loops tend to slightly dampen the destabilising effect of nutrient enrichment on consumer temporal variability while they have opposite effects for primary producers. By considering nutrient cycling, this new model improves our understanding of the response of food webs to nutrient availability and opens perspectives to better link studies on food web dynamics and ecosystem functioning.

Effects of warming, climate extremes and phosphorus enrichment on the growth, sexual reproduction and propagule carbon and nitrogen stoichiometry of Potamogeton crispus L

Contemporary evidence suggests that submerged macrophytes are experiencing a global decline due to the multiple compounding anthropogenic stressors impacting shallow lake ecosystems. Eutrophication and climate change are two main widespread, often co-occurring stressors, yet evidence concerning their interactive effects on aquatic plants remains partial and fragmented. Predicting the response of submerged aquatic vegetation to the combined effects of nutrient pollution and compound climate warming (mean + variability) is therefore crucial for the conservation and management of these valuable and vulnerable ecosystems. Here, we present the results of an outdoor mesocosm experiment examining the combined effects of nutrient enrichment (phosphorus addition) and warming (a 4 °C increase in mean temperature above present ambient conditions applied as either a constant increase or a variable increase ranging between 0 and 8 °C to mimic the effect of extreme events but keeping an equivalent total amount of warming) on Potamogeton crispus L. Warming accelerated the growth and senescence of P. crispus suggesting a more important role in maintaining the clear water state in winter-early spring but concomitant to possible earlier turbid states in summer. Warming also consistently advanced the flowering phenology but had no significant effect on flowering duration. There were no significant differences in the life cycle between the two warming treatments, while phosphorus addition also had little effect. However, under phosphorus enrichment, P. crispus increased sexual reproduction investment producing higher seed setting rate per infructescence. In contrast, warming, especially variable warming, may decrease sexual reproduction investment by reducing the number of infructescences. Seed and turion stoichiometry were altered by the combination of warming and phosphorus addition, but the changes were complex and difficult to interpret.