Changes In Nutrient Stoichiometry Research Articles

Wastewater microorganisms impact microbial diversity and important ecological functions of stream periphyton

Effluents of wastewater treatment plants can impact microbial communities in the receiving streams. However, little is known about the role of microorganisms in wastewater as opposed to other wastewater constituents, such as nutrients and micropollutants. We aimed therefore at determining the impact of wastewater microorganisms on the microbial diversity and function of periphyton, key microbial communities in streams. We used a flow-through channel system to grow periphyton upon exposure to a mixture of stream water and unfiltered or ultra-filtered wastewater. Impacts were assessed on periphyton biomass, activities and tolerance to micropollutants, as well as on microbial diversity. Our results showed that wastewater microorganisms colonized periphyton and modified its community composition, resulting for instance in an increased abundance of Chloroflexi and a decreased abundance of diatoms and green algae. This led to shifts towards heterotrophy, as suggested by the changes in nutrient stoichiometry and the increased mineralization potential of carbon substrates. An increased tolerance towards micropollutants was only found for periphyton exposed to unfiltered wastewater but not to ultra-filtered wastewater, suggesting that wastewater microorganisms were responsible for this increased tolerance. Overall, our results highlight the need to consider the role of wastewater microorganisms when studying potential impacts of wastewater on the receiving water body.

Harmful algal blooms and cyanotoxins in Lake Amatitlán, Guatemala, coincided with ancient Maya occupation in the watershed

Human-induced deforestation and soil erosion were environmental stressors for the ancient Maya of Mesoamerica. Furthermore, intense, periodic droughts during the Terminal Classic Period, ca. Common Era 830 to 950, have been documented from lake sediment cores and speleothems. Today, lakes worldwide that are surrounded by dense human settlement and intense riparian land use often develop algae/cyanobacteria blooms that can compromise water quality by depleting oxygen and producing toxins. Such environmental impacts have rarely been explored in the context of ancient Maya settlement. We measured nutrients, biomarkers for cyanobacteria, and the cyanotoxin microcystin in a sediment core from Lake Amatitlán, highland Guatemala, which spans the last ∼2,100 y. The lake is currently hypereutrophic and characterized by high cyanotoxin concentrations from persistent blooms of the cyanobacterium Microcystis aeruginosa Our paleolimnological data show that harmful cyanobacteria blooms and cyanotoxin production occurred during periods of ancient Maya occupation. Highest prehistoric concentrations of cyanotoxins in the sediment coincided with alterations of the water system in the Maya city of Kaminaljuyú, and changes in nutrient stoichiometry and maximum cyanobacteria abundance were coeval with times of greatest ancient human populations in the watershed. These prehistoric episodes of cyanobacteria proliferation and cyanotoxin production rivaled modern conditions in the lake, with respect to both bloom magnitude and toxicity. This suggests that pre-Columbian Maya occupation of the Lake Amatitlán watershed negatively impacted water potability. Prehistoric cultural eutrophication indicates that human-driven nutrient enrichment of water bodies is not an exclusively modern phenomenon and may well have been a stressor for the ancient Maya.

Spatial pattern of C:N:P stoichiometry characteristics of alpine grassland in the Altunshan Nature Reserve at North Qinghai-Tibet Plateau

Carbon (C), nitrogen (N) and phosphorus (P) are the most important elemental components of an ecosystem. Nutrient stoichiometry reflects their cycling and distribution with ecosystems. The changes in nutrient stoichiometry in different ecosystems are not consistent. The altitudinal gradient within the Altunshan Nature Reserve can be used as a natural experiment to verify the effect of different altitudes gradients on nutrients changes. Therefore, we studied the changes in plant and soil nutrients from alpine desert ecosystem to alpine grassland ecosystem in this region. The results showed that there was a linear correlation for plant C, N and P, as well as soil C and N with altitude gradient. The C: N and C: P of plants and C: P and N: P of soils were linearly correlated with the elevation gradient. Plant C and N were mainly affected by soil C and N content, but there was no significant correlation between plant P and soil nutrient concentration. Soil nutrients were affected by soil moisture content and pH. Soil moisture content increased gradually with altitude, promoting nutrient mineralization and availability to the ecosystem. The increase of soil nutrient availability and soil water content significantly increased the species diversity, promoting the plant community assembly and the succession from alpine desert ecosystem to alpine grassland ecosystem. This study analyzed the relationship between nutrient cycling and distribution in different ecosystem types on the Qinghai-Tibet Plateau and found that nutrient changes were greatly affected by soil moisture in the arid and cold regions.

Global Dam‐Driven Changes to Riverine N:P:Si Ratios Delivered to the Coastal Ocean

AbstractRiver damming alters nutrient fluxes along the land‐ocean aquatic continuum as a result of biogeochemical processes in reservoirs. Both the changes in riverine nutrient fluxes and nutrient ratios impact ecosystem functioning of receiving water bodies. We utilize spatially distributed mechanistic models of nitrogen (N), phosphorus (P), and silicon (Si) cycling in reservoirs to quantify changes in nutrient stoichiometry of river discharge to coastal waters. The results demonstrate that the growing number of dams decouples the riverine fluxes of N, P, and Si. Worldwide, preferential removal of P over N in reservoirs increases N:P ratios delivered to the ocean, raising the potential for P limitation of coastal productivity. By midcentury, more than half of the rivers discharging to the coastal zone will experience a higher removal of reactive Si relative to reactive P and total N, in response to the rapid pace at which new hydroelectric dams are being built.

Global Change Impacts on Forest Soils: Linkage Between Soil Biota and Carbon-Nitrogen-Phosphorus Stoichiometry

Forest ecosystems are subjected to global change drivers worldwide, such as increasing temperature, atmospheric carbon dioxide, nutrient pollution, as well as changes in fire and precipitation regimes. These global change drivers have greatly modified the biogeochemical cycles of carbon (C), nitrogen (N), and phosphorus (P), which has an impact on primary productivity in forest ecosystems and in turn, affect the quality and quantity of resources entering the soil food web. However, C, N, and P soil dynamics have been mostly studied without considering their coupling effects on soil organisms. This is of critical interest because changes in nutrient stoichiometry may have a strong effect on soil biota and the ecosystem functions they drive. Further, most studies have focused on global change effects on bacteria and fungi and their C:N:P stoichiometry, while neglecting other soil organisms at higher trophic levels. This has led to an incomplete understanding of how the entire soil food web drives ecosystem processes involved in organic matter turnover and nutrient cycling. Here, we review studies that investigated how global change drivers impact C:N:P stoichiometry of soil organisms at different trophic levels in forest ecosystems and identify important knowledge gaps. We propose future directions for research on global change impacts on the linkages between soil biota and C:N:P stoichiometry.

Nutrient stoichiometry (N:P) controls nitrogen fixation and distribution of diazotrophs in a tropical eutrophic estuary

Nitrogen fixation and its ecological regulation are poorly understood in the tropical estuaries, which are highly influenced by anthropogenic disturbances. In this study, we investigated the role of nutrient stoichiometry in the diversity, abundance and activity of N2-fixing bacterial community and their seasonal variations in the water column of a tropical eutrophic estuary (Cochin estuary). The N2 fixation rates in the estuary ranged from 0.1 to 2.0 nmol N2 l−1 h−1, with higher activity during post-monsoon and lower during monsoon. The rates are appeared to be primarily controlled by dissolved inorganic nitrogen and phosphorous (N:P) ratio. Clone library analysis of nitrogenase (nifH) gene revealed that the major N2 fixing phylotypes belong to Cluster I and Cluster III diazotrophs. The overall findings of this study suggest that monsoon induced seasonal changes in nutrient stoichiometry control the distribution and activity of diazotrophs in a tropical estuary.

Ecoenzymatic stoichiometry reveals microbial phosphorus limitation decreases the nitrogen cycling potential of soils in semi-arid agricultural ecosystems

Variations in soil microbial metabolism currently represent one of the greatest areas of uncertainty with regard to soil nutrient cycles and the control of terrestrial carbon (C) and nitrogen (N) loss and are poorly understood in agricultural ecosystems with intensive farming practices. In this study, extracellular enzymatic stoichiometry models and quantitative PCR techniques were used to examine microbial metabolic limitation and its relationship with N-cycling gene expression in semi-arid agricultural ecosystems considering four N fertilization levels (N 0, N 100, N 250, and N 400 kg N ha−1) and two agronomic strategies (film mulching and no mulching). Film mulching increased microbial C limitation (reflecting microbial C metabolism size; 0.189 of the total effects), while very small effects on microbial phosphorus (P) limitation were observed (-0.007 of the total effects). N fertilization increased the microbial demand for P (microbial P limitation; 0.504 of the total effects). Increased microbial C metabolism was mainly attributed to increased soil moisture content after film mulching, which enhanced microbial decomposition of organic C (high C-acquiring enzyme activities). Changes in nutrient stoichiometry and the increase in N availability due to N fertilization were largely responsible for increased microbial P limitation. Furthermore, microbial P limitation negatively affected the abundance of AOA amoA, AOB amoA (involved in nitrification), nirK, nirS, nosZ (involved in denitrification) genes, strongly inhibiting nitrification and denitrification potential (-0.743 and -0.761 of the total effects, respectively). The present results suggest that agricultural ecosystems with film mulching are conducive to organic residue decomposition, while appropriate P limitation under N fertilization could reduce the loss of N due to nitrification and denitrification in soil. This study highlights the importance of elemental stoichiometry-driven microbial metabolic variation in understanding soil nutrient cycles and optimizing agricultural practices.

Plant sizes mediate mowing-induced changes in nutrient stoichiometry and allocation of a perennial grass in semi-arid grassland.

While mowing‐induced changes in plant traits and their effects on ecosystem functioning in semi‐arid grassland are well studied, the relations between plant size and nutrient strategies are largely unknown. Mowing may drive the shifts of plant nutrient limitation and allocation. Here, we evaluated the changes in nutrient stoichiometry and allocation with variations in sizes of Leymus chinensis, the dominant plant species in Inner Mongolia grassland, to various mowing frequencies in a 17‐yr controlled experiment. Affected by mowing, the concentrations of nitrogen (N), phosphorus (P), and carbon (C) in leaves and stems were significantly increased, negatively correlating with plant sizes. Moreover, we found significant trade‐offs between the concentrations and accumulation of N, P, and C in plant tissues. The N:P ratios of L. chinensis aboveground biomass, linearly correlating with plant size, significantly decreased with increased mowing frequencies. The ratios of C:N and C:P of L. chinensis individuals were positively correlated with plant size, showing an exponential pattern. With increased mowing frequencies, L. chinensis size was correlated with the allocation ratios of leaves to stems of N, P, and C by the tendencies of negative parabola, positive, and negative linear. The results of structure equation modeling showed that the N, P, and C allocations were co‐regulated by biomass allocation and nutrient concentration ratios of leaves to stems. In summary, we found a significant decoupling effect between plant traits and nutrient strategies along mowing frequencies. Our results reveal a mechanism for how long‐term mowing‐induced changes in concentrations, accumulations, ecological stoichiometry, and allocations of key elements are mediated by the variations in plant sizes of perennial rhizome grass.

Plant and soil nutrient stoichiometry along primary ecological successions: Is there any link?

Ecological stoichiometry suggests that plant Nitrogen (N)-to-Phosphorus (P) ratios respond to changes in both soil N:P stoichiometry and soil N and P availability. Thus we would expect that soil and plant N:P ratios be significantly related along natural gradients of soil development such as those associated with primary ecological successions. Here we explicitly search for linkages between plant and soil N:P stoichiometry along four primary successions distributed across Europe. We measured N and P content in soils and plant compartments (leaf, stem and root) of 72 wild plant species distributed along two sand dune and two glacier successions where soil age ranges from few to thousand years old. Overall we found that soil N:P ratios strongly increased along successional stages, however, plant N:P ratios were neither related to soil N:P stoichiometry nor to changes in soil N and P availability. Instead changes in plant nutrient stoichiometry were “driven” by plant-functional-group identity. Not only N:P ratios differed between legumes, grasses and forbs but each of these plant functional groups maintained N:P ratios relatively constant across pioneer, middle and advanced successional stages. Our evidence is that soil nutrient stoichiometry may not be a good predictor of changes in plant N:P stoichiometry along natural primary ecological successions, which have not reached yet a retrogressive stage. This could be because wild-plants rely on mechanisms of internal nutrient regulation, which make them less dependent to changes in soil nutrient availability under unpredictable environmental conditions. Further studies need to clarify what underlying evolutionary and eco-physiological mechanisms determine changes in nutrient stoichiometry in plant species distributed across natural environmental gradients.

Changes in nutrient stoichiometry, elemental homeostasis and growth rate of aquatic litter-associated fungi in response to inorganic nutrient supply.

Aquatic fungi mediate important energy and nutrient transfers in freshwater ecosystems, a role potentially altered by widespread eutrophication. We studied the effects of dissolved nitrogen (N) and phosphorus (P) concentrations and ratios on fungal stoichiometry, elemental homeostasis, nutrient uptake and growth rate in two experiments that used (1) liquid media and a relatively recalcitrant carbon (C) source and (2) fungi grown on leaf litter in microcosms. Two monospecific fungal cultures and a multi-species assemblage were assessed in each experiment. Combining a radioactive tracer to estimate fungal production (C accrual) with N and P uptake measurements provided an ecologically relevant estimate of mean fungal C:N:P of 107:9:1 in litter-associated fungi, similar to the 92:9:1 obtained from liquid cultures. Aquatic fungi were found to be relatively homeostatic with respect to their C:N ratio (~11:1), but non-homeostatic with respect to C:P and N:P. Dissolved N greatly affected fungal growth rate and production, with little effect on C:nutrient stoichiometry. Conversely, dissolved P did not affect fungal growth and production but controlled biomass C:P and N:P, probably via luxury P uptake and storage. The ability of fungi to immobilize and store excess P may alter nutrient flow through aquatic food webs and affect ecosystem functioning.

Effects of silver nanoparticles on bacterioplankton in a boreal lake

Summary Silver nanoparticles (AgNPs) are widely used antimicrobial agents and a growing body of evidence suggests that their release into aquatic environments threatens natural bacterial communities and whole ecosystems. However, a knowledge gap exists between the toxic effects of AgNPs found in laboratory studies and their potential impacts in natural environments. In an enclosure experiment conducted in a boreal lake, we exposed natural bacterial communities to AgNPs with two common types of coatings (polyvinylpyrrolidone (PVP) and citrate) under two different exposure regimes, a one‐time (pulse) and a continuous (chronic) addition. AgNP additions increased Ag concentrations to nearly 50 μg L−1 in the highest treatments. We examined bacterial responses (abundance, biomass, production, chlorophyll‐a content and nutrient stoichiometry) over the course of 6 weeks in the summer of 2012. Bacterioplankton exposed to AgNPs initially accumulated Ag over the experimental period regardless of AgNP concentration or coating. After the initial period of increase, Ag in the bacterial size fraction changed largely in concert with bacterial biomass. We found no toxic effects of AgNPs on bacterioplankton abundance, biomass, production or chlorophyll‐a content throughout the experiment. Bacterial production was greater after the pulse addition of PVP‐coated AgNPs and in the chronic addition of PVP‐coated AgNPs at the highest concentrations. Furthermore, AgNPs produced no significant changes in nutrient stoichiometry of the bacterioplankton size fraction. This lack of effects of AgNPs on lake bacterioplankton observed under the natural conditions studied here differs from results of short‐term and laboratory studies of single‐species bacterial cultures. Our results thus indicate AgNP effects in lakes may be less than expected based on standard laboratory experiments, and that additional studies are needed to understand AgNP toxicity under realistic natural conditions in lakes and other ecosystems.

Seasonal Alternation Between Groundwater Discharge and Benthic Coupling as Nutrient Sources in a Shallow Coastal Lagoon

Submarine groundwater discharge (SGD) is now recognized as an important source of nutrients and freshwater to some coastal environments. We studied a shallow coastal lagoon (Little Lagoon, AL, USA) in the northern Gulf of Mexico that lacks riverine inputs but has been suspected to receive significant SGD. We observed persistent salinity gradients between the east and west ends of the lagoon and the pass connecting it to the Gulf of Mexico. Covariance between salinity in the lagoon and the groundwater tracer 222Rn indicated that SGD was responsible for the salinity gradients and is the primary source of freshwater to the lagoon. Cluster analysis of 246 biweekly samples based on temperature, salinity, and two proxies of SGD revealed two hydrographic regimes with different drivers for nutrient inputs. In samples characterized by high discharge and low temperatures (generally December–April), total nitrogen (TN) was negatively correlated with salinity, while total phosphorus (TP) was positively correlated with temperature. Total nitrogen in the groundwater was very high (0.36–4.80 mM) while total phosphorus was relatively low (0.3–2.3 μM), consistent with SGD as the source of TN during the high-discharge periods. In periods with low discharge and higher temperatures (approx. May–November), TN and TP had strong positive correlations with temperature and are inferred to originate from benthic efflux. Seasonal changes in nutrient stoichiometry in the lagoon water column also indicate an alternation between low TN/TP sediments and high TN/TP groundwater as the primary sources of nitrogen in this system.

Mariculture: significant and expanding cause of coastal nutrient enrichment

Mariculture (marine aquaculture) generates nutrient waste either through the excretion by the reared organisms, or through direct enrichment by, or remineralization of, externally applied feed inputs. Importantly, the waste from fish or shellfish cannot easily be managed, as most is in dissolved form and released directly to the aquatic environment. The release of dissolved and particulate nutrients by intensive mariculture results in increasing nutrient loads (finfish and crustaceans), and changes in nutrient stoichiometry (all mariculture types). Based on different scenarios, we project that nutrients from mariculture will increase up to six fold by 2050 with exceedance of the nutrient assimilative capacity in parts of the world where mariculture growth is already rapid. Increasing nutrient loads and altered nutrient forms (increased availability of reduced relative to oxidized forms of nitrogen) and/or stoichiometric proportions (altered nitrogen:phosphorus ratios) may promote an increase in harmful algal blooms (HABs) either directly or via stimulation of algae on which mixotrophic HABs may feed. HABs can kill or intoxicate the mariculture product with severe economic losses, and can increase risks to human health.

A combined geochemical and hydrological approach for understanding macronutrient sources

This study employed complementary geochemical techniques and distributed hydrological modelling to investigate multiple sources of catchment macronutrients and characterise their changes in contrasting storm and baseflow conditions. This approach was demonstrated for the Beult catchment in the county of Kent (England), a designated Site of Special Scientific Interest (SSSI) indentified as failing to meet water quality standards for key nutrients under the Water Framework Directive. Significant changes in nutrient stoichiometry and bioavailability are observed for surface waters under contrasting flow regimes. Soluble reactive phosphorus (SRP) concentrations are approximately twice as high during baseflow compared to high flow, while the inverse is true for particulate, colloidal and dissolved hydrolysable phosphorus, dissolved organic carbon and nitrate. Nitrogen (N):phosphorus (P) ratios are lower during baseflow for most surface waters impacted by diffuse sources of pollution. Fluorescence indices of dissolved organic matter (DOM) show that waste water inputs may be locally important sources of more complex low molecular weight DOM, particularly during baseflow. Nitrate N and O isotope signatures, combined with other dissolved chemical tracers, confirm the dominance of wastewater N inputs at sites downsteam of sewerage treatment works during baseflow, with a shift towards the soil N pool in surface waters across the catchment during high flow. Distributed hydrological modelling using the Grid-to-Grid model reveal areas with the greatest runoff also export higher N and P concentrations, and hence deliver a greater flux of macronutrients, while forested areas with low nutrient concentrations reduce runoff and nutrient fluxes. During periods of high runoff, nested sampling indicates that nutrient fluxes scale with catchment area. This combined approach enables a more thorough assessment of the macronutrient sources and dynamics, better informing management options in nutrient impacted catchments.

Effect of storm events on riverine nitrogen dynamics in a subtropical watershed, southeastern China

Rain storms are predicted to increase in the subtropical region due to climate change. However, the effects of storm events on riverine nitrogen (N) dynamics are poorly understood. In this study, the riverine N dynamics and storm effects in a large subtropical river (North Jiulong River, southeastern China) were investigated through continuous sampling of two storm events which occurred in June 2010 and June 2011. The results disclosed a strong linkage between N dynamics and hydrological controls and watershed characteristics. The extreme storm in June 2010 resulted in more fluctuations in N concentrations, loads, and composition, compared with the moderate storm in June 2011. There were contrasting patterns (e.g., the hysteresis effect) between nitrate and ammonium behavior in storm runoff, reflecting their different supply source and transport mechanism. Overall, nitrate supply originated from subsurface runoff and was dominated by within-channel mobilization, while ammonium was mainly from over-land sources and flushed by surface runoff. Extreme storm runoff (2010) caused a four-fold increase in dissolved inorganic N fluxes (DIN), with a greater fraction of ammonium (up to 30% of DIN) compared with the moderate storm and background flow condition (less than 15%). Storm-driven sharp increases of N loads and changes in nutrient stoichiometry (more ammonium) might have been connected with algal blooms in the adjacent estuary and Xiamen Bay. Combined with the background flow measurement of N gradients along the main river and a stream together with anthropogenic N load information, the interactive effect of hydrological and biogeochemical process on riverine N was preliminarily revealed. Current results suggested that storm runoff N was controlled by rainfall, hydrological condition, antecedent soil moisture, spatial variability of land-based N source, and damming. These findings could be used as a reference for future water quality monitoring programs and the development of a pollution mitigation strategy.

Phytoplankton community dynamics during late spring coccolithophore blooms at the continental margin of the Celtic Sea (North East Atlantic, 2006–2008)

We determined the spatial and temporal dynamics of major phytoplankton groups in relation to biogeochemical and physical variables during the late spring coccolithophore blooms (May–June) along and across the continental margin in the Celtic Sea (2006–2008). Photosynthetic biomass (chl a) of the dominant plankton groups was determined by CHEMTAX analysis of chromatographic (HPLC) pigment signatures.Phytoplankton standing stock biomass varied substantially between and during the campaigns (areal chl a [mgchl am−2] in June 2006: 63.8±26.5, May 2007: 27.9±8.4, and May 2008: 41.3±21.8), reflecting the different prevailing conditions of weather, irradiance, and sea surface temperature between the campaigns. Coccolithophores, represented mainly by Emiliania huxleyi, and diatoms were the dominant phytoplankton groups, with a maximal contribution of, respectively, 72% and 89% of the total chl a. Prasinophytes, dinoflagellates, and chrysophytes often co-occurred during coccolithophorid blooms, while diatoms dominated the phytoplankton biomass independently of the biomass of other groups. The location of the stations on the shelf or on the slope side of the continental margin did not influence the biomass and the composition of the phytoplankton community despite significantly stronger water column stratification and lower nutrient concentrations on the shelf. The alternation between diatom and coccolithophorid blooms of similar biomass, following the mostly diatom-dominated main spring bloom, was partly driven by changes in nutrient stoichiometry (N:P and dSi:N). High concentrations of transparent exopolymer particles (TEP) were associated with stratified, coccolithophore-rich water masses, which probably originated from the slope of the continental margin and warmed during advection onto the shelf. Although we did not determine the proportion of export production attributed to phytoplankton groups, the abundance of coccolithophores, together with TEP and coccoliths may affect the carbon export efficiency through increased sinking rates of particles formed by aggregation of TEP and coccoliths.

Global Hindcasts and Future Projections of Coastal Nitrogen and Phosphorus Loads Due to Shellfish and Seaweed Aquaculture

A model was developed to estimate nitrogen and phosphorus budgets for aquaculture production of crustaceans, bivalves, gastropods, and seaweed, using country production data for the 1970–2006 period from the Food and Agriculture Organi- zation and scenarios based on the Millenium Assessment for 2006–2050. Global production of crustaceans (18% yr−1), molluscs (7.4%), and seaweed (8%) increased rapidly during the 1970–2006 period. Scenarios indicate that annual nutrient release from all shellfish (crustaceans, bivalves, and gastropods) aquaculture will rapidly grow from 0.4 to up to 1.7 million tonnes of nitrogen and from 0.01 to 0.3 million tonnes of phosphorus between 2006 and 2050. The nitrogen and phosphorus releases from global freshwater shellfish aquaculture will increase from 1% of river export in 2006 to up to 6% in 2050. Marine shellfish production is an important contributor to nutrient loading of coastal seas, particularly in Eastern Asia. Nitrogen (7% of marine aquaculture + river export in 2006 and up to 19% in 2050) and phosphorus (12% in 2006 and up to 30% in 2050) releases from Chinese marine shellfish aquaculture are important and growing contributors to total nutrient inputs to coastal seas. Production of crustaceans and bivalves causes changes in nutrient stoichiometry and increasing reduced and organic nitrogen forms, which are of concern because of their preferential use by some harmful algae. Nutrient withdrawal by seaweed is projected to increase rapidly over the coming decades. To overcome effects of increasing nutrient release from shellfish production, integrated systems that include seaweed may play an important role in reducing this nutrient load. [Supplementary materials are available for this article. Go to the publisher's online edition of Reviews in Fisheries Science for the following free supplemental resources: FAO data on shellfish and seaweed production, and model results for the different nitrogen and phosphorus budget terms calculated for 1970–2006 and for 2006–2050 for the four Millennium Ecosystem Assessment scenarios]

Ecological Stoichiometry, Biogeochemical Cycling, Invasive Species, and Aquatic Food Webs: San Francisco Estuary and Comparative Systems

Eutrophication has altered food webs across aquatic systems, but effects of nutrient stoichiometry (varying nutrient ratios) on ecosystem structure and function have received less attention. A prevailing assumption has been that nutrients are not ecologically relevant unless concentrations are limiting to phytoplankton. However, changes in nutrient stoichiometry fundamentally affect food quality at all levels of the food web. Here, 30-year records of nitrogen and phosphorus concentrations and ratios, phytoplankton, zooplankton, macroinvertebrates, and fish in the San Francisco Estuary (Bay Delta) were examined to collectively interpret ecosystem changes within the framework of ecological stoichiometry. Changes in nutrient concentrations and nutrient ratios over time fundamentally affect biogeochemical nutrient dynamics that can lead to conditions conducive to invasions of rooted macrophytes and bivalve molluscs, and the harmful cyanobacterium Microcystis. Several other aquatic ecosystems considered here have exhibited similar changes in food webs linked to stoichiometric changes. Nutrient stoichiometry is thus suggested to be a significant driver of food webs in the Bay Delta by altering food quality and biogeochemical dynamics. Since nitrogen-to-phosphorus ratios have increased over time, an overall implication is that remediation of fish populations in the San Francisco Estuary will require significant nitrogen reductions to restore the historic ecological stoichiometric balance and the food web.