Effects Of Nutrient Enrichment Research Articles

Nutrient enrichment and flow regulation impair structure and function of a large river as revealed by aquatic hyphomycete species richness, biomass, and decomposition rates

AbstractDiversity, sporulation rates, and biomass of aquatic hyphomycetes (Kingdom Fungi) on leaf litter were examined to assess the effects of nutrient enrichment and flow regulation on the ecological integrity of the Saint John River, New Brunswick, Canada. Leaf decomposition rate was used as an indicator of ecological function, whereas fungal abundance and diversity were chosen as structural indicators. Leaf-litter bags of yellow birch (Betula alleghaniensis) were deployed at 8 sites, periodically retrieved, and analyzed during 2 consecutive years in unregulated and regulated reaches. The interaction between nutrient enrichment and flow regulation regime significantly affected leaf decomposition rate and fungal sporulation during summer and autumn and fungal diversity during autumn. Flow regulation regime was the primary driver for fungal diversity during summer. In contrast, nutrient enrichment significantly affected fungal biomass. In general, this large 7th-order river responded to flow regulation a...

Cumulative effects of multiple stressors: An invasive oyster and nutrient enrichment reduce subsequent invasive barnacle recruitment

Studies identifying interactions between biological invasions and other stressors have generally focussed on quantifying their cumulative effects on mature species assemblages. In benthic systems, however, early life history processes are key determinants of assemblage structure and functioning. This study tested whether the presence of an invasive species affected early life history processes of two common barnacle species and whether this was affected by a second common stressor, nutrient enrichment. The results of a field experiment identified and characterised the effects of an invasive oyster, Crassostrea gigas, on the early life history processes of the two barnacle species under ambient and enriched nutrient conditions. In the presence C. gigas, the invasive barnacle Austrominius modestus, had a lower recruitment rate, however, there was no effect of the presence of C. gigas on native barnacle, Semibalanus balanoides, recruitment. Nutrient enrichment also reduced the recruitment rate of A. modestus, however, there was no evidence of synergistic or antagonistic interactions between these stressors, indicating their cumulative effects were additive. There was no effect of nutrient enrichment on native barnacle recruitment. Our results show that the presence of an invasive oyster and nutrient enrichment altered the recruitment of another non-native benthic species. These findings emphasise the importance of considering early life history processes when assessing effects of multiple stressors on communities.

Seasonal regulation of herbivory and nutrient effects on macroalgal recruitment and succession in a Florida coral reef.

Herbivory and nutrient enrichment are drivers of benthic dynamics of coral reef macroalgae; however, their impact may vary seasonally. In this study we evaluated the effects of herbivore pressure, nutrient availability and potential propagule supply on seasonal recruitment and succession of macroalgal communities on a Florida coral reef. Recruitment tiles, replaced every three months, and succession tiles, kept in the field for nine months, were established in an ongoing factorial nutrient enrichment-herbivore exclusion experiment. The ongoing experiment had already created very different algal communities across the different herbivory and nutrient treatments. We tracked algal recruitment, species richness, and species abundance through time. Our results show seasonal variation in the effect of herbivory and nutrient availability on recruitment of coral reef macroalgae. In the spring, when there was higher macroalgal species richness and abundance of recruits, herbivory appeared to have more control on macroalgal community structure than did nutrients. In contrast, there was no effect of either herbivory or nutrient enrichment on macroalgal communities on recruitment tiles in cooler seasons. The abundance of recruits on tiles was positively correlated with the abundance of algal in the ongoing, established experiment, suggesting that propagule abundance is likely a strong influence on algal recruitment and early succession. Results of the present study suggest that abundant herbivorous fishes control recruitment and succession of macroalgae, particularly in the warm season when macroalgal growth is higher. However, herbivory appears less impactful on algal recruitment and community dynamics in cooler seasons. Ultimately, our data suggest that the timing of coral mortality (e.g., summer vs. winter mortality) and freeing of benthic space may strongly influence the dynamics of algae that colonize open space.

Biotic resistance buffers the effects of nutrient enrichment on the success of a highly invasive aquatic plant

Summary Biotic resistance theory predicts that the presence of a species‐rich native community is able to inhibit the establishment of invasive species. On the other hand, disturbed environments, for example, through eutrophication, tend to be more easily invaded. We investigated the effect of biotic resistance on the establishment success of the aquatic grass Urochloa arrecta in nutrient‐enriched and non‐enriched mesocosms so as to test the hypothesis that the effects of biotic resistance on invader success depend on ecosystem trophic state. Invader success, measured as plant biomass and length, increased with nutrient enrichment in treatments without biotic resistance, whereas there was no difference between the success of the invader in enriched or non‐enriched mesocosms in the presence of biotic resistance. Our results suggest that eutrophication enhanced the resistance offered by the native community and thereby reduced the establishment of invaders. The results underscore the importance of native communities as biotic filters to invasion, especially in environments that are subjected to anthropogenic impacts such as nutrient enrichment.

Impact of contaminated sediment elutriate on coastal phytoplankton community (Thau lagoon, Mediterranean Sea, France)

Effects of sediment-released contaminants and nitrogen were assessed on phytoplankton communities sampled from Thau lagoon (France, Mediterranean Sea) and one close offshore marine station. Phytoplankton was exposed to sediment elutriate (seawater containing a mix of metals, organic chemicals, and nutrients) or to ammonium enrichment for four days using immersed microcosms exposed to natural conditions of light and temperature. Functional (production - respiration balance) and structural (taxonomy and cell densities) responses of the phytoplankton community were assessed. In the lagoon, both treatments stimulated phytoplankton growth, compare to controls. Conversely in the offshore station, the phytoplankton growth was stimulated only with the sediment elutriate addition. In offshore and lagoon stations, both treatments caused a shift in the taxonomic composition of the phytoplankton. Proliferation of potentially toxic diatoms and dinoflagellates resulted from the addition of elutriate. Correspondence analysis determined that phytoplankton from the offshore station was more sensitive to both treatments compared to the lagoon community. According to daily production and respiration balance, lagoon community metabolism remained heterotrophic (P<R) for all treatments, whereas only transient shifts to net autotrophy (P>R) were observed in the offshore community. Direct toxicity of contaminants released from sediment, if any, was therefore masked by nutrient enrichment effects, whereas indirect evidence of contaminant pressure was highlighted by changes in community composition and metabolism.

Short-term interactive effects of ultraviolet radiation, carbon dioxide and nutrient enrichment on phytoplankton in a shallow coastal lagoon

The main goal of this study was to evaluate short-term interactions between increased CO2, UVR and inorganic macronutrients (N, P and Si) on summer phytoplankton assemblages in the Ria Formosa coastal lagoon (SW Iberia), subjected to intense anthropogenic pressures and highly vulnerable to climate change. A multifactorial experiment using 20 different nutrient-enriched microcosms exposed to different spectral and CO2 conditions was designed. Before and after a 24-h in situ incubation, phytoplankton abundance and composition were analysed. Impacts and interactive effects of high CO2, UVR and nutrients varied among different functional groups. Increased UVR had negative effects on diatoms and cyanobacteria and positive effects on cryptophytes, whereas increased CO2 inhibited cyanobacteria but increased cryptophyte growth. A positive synergistic interaction between CO2 and UVR was observed for diatoms; high CO2 counteracted the negative effects of UVR under ambient nutrient concentrations. Nutrient enrichments suppressed the negative effects of high CO2 and UVR on cyanobacteria and diatoms, respectively. Beneficial effects of CO2 were observed for diatoms and cryptophytes under combined additions of nitrate and ammonium, suggesting that growth may be limited by DIC availability when the primary limitation by nitrogen is alleviated. Beneficial effects of high CO2 and UVR in diatoms were also induced or intensified by ammonium additions.

Consumer-mediated nutrient recycling is influenced by interactions between nutrient enrichment and the antimicrobial agent triclosan

The fate and transport of triclosan (5-chloro-2-[2,4-dichlorophenoxy]phenol), a widely used antimicrobial agent in personal care products, in aquatic ecosystems is a growing environmental concern. At ecosystem scales, triclosan potentially interacts with co-occurring nutrient stressors to affect overall biogeochemical cycling through consumer-mediated nutrient recycling pathways. We examined N- and P-excretion rates of snails (Physella spp.) in 12 outdoor experimental stream mesocosms dosed with 3 P treatments crossed with 3 triclosan treatments and a methanol carrier control. Snail N- and P-excretion rates increased with decreasing periphyton C∶N and C∶P ratios across the P-enrichment gradient. N- and P-excretion rates were significantly higher in the high-triclosan than in the methanol control treatments on day 14, but only in high-P-enrichment streams. However, methanol had positive effects on N- and P-excretion rates in low- and high-P-enrichment treatments compared to nonsolvent controls, but no effect at background P concentrations. Multiple inferences can be drawn from our study. First, whereas many investigators have confirmed that using methanol as a carrier below established regulatory levels does not influence laboratory and mesocosm ecotoxicology responses, our results suggest that low-level methanol concentrations can influence measures of ecosystem function. Second, higher use of triclosan and other antimicrobial agents in commercial products coupled with predicted urbanization and surface-water shortages in the future have the potential to increase triclosan concentrations and shift patterns in consumer-mediated nutrient cycling in aquatic systems because of interactive effects of antimicrobial agents and nutrient enrichment.

Convergence of detrital stoichiometry predicts thresholds of nutrient-stimulated breakdown in streams.

Nutrient enrichment of detritus-based streams increases detrital resource quality for consumers and stimulates breakdown rates of particulate organic carbon (C). The relative importance of dissolved inorganic nitrogen (N) vs. phosphorus (P) for detrital quality and their effects on microbial- vs. detritivore-mediated detrital breakdown are poorly understood. We tested effects of experimental N and P additions on detrital stoichiometry (C:N, C:P) and total and microbial breakdown (i.e., with and without detritivorous shredders, respectively) of five detritus types (four leaf litter species and wood) with different initial C : nutrient content. We enriched five headwater streams continuously for two years at different relative availabilities of N and P and compared breakdown rates and detrital stoichiometry to pretreatment conditions. Total breakdown rates increased with nutrient enrichment and were predicted by altered detrital stoichiometry. Streamwater N and P, fungal biomass, and their interactions affected stoichiometry of detritus. Streamwater N and P decreased detrital C:N, whereas streamwater P had stronger negative effects on detrital C:P. Nutrient addition and fungal biomass reduced C:N by 70% and C:P by 83% on average after conditioning, compared to only 26% for C:N and 10% for C:P under pretreatment conditions. Detritus with lowest initial nutrient content changed the most and had greatest increases in total breakdown rates. Detrital stoichiometry was reduced and differences among detritus types were homogenized by nutrient enrichment. With enrichment, detrital nutrient content approached detritivore nutritional requirements and stimulated greater detritivore vs. microbial litter breakdown. We used breakpoint regression to estimate values of detrital stoichiometry that can potentially be used to indicate elevated breakdown rates. Breakpoint ratios for total breakdown were 41 (C:N) and 1518 (C:P), coinciding with total breakdown rates that were ~1.9times higher when C:N or C:P fell below these breakpoints. Microbial and shredder-mediated breakdown rates both increased when C:N and C:P were reduced, suggesting that detrital stoichiometry is useful for predicting litter breakdown dominated by either microbial or shredder activity. Our results show strong effects of nutrient enrichment on detrital stoichiometry and offer a robust link between a potential holistic nutrient loading metric (decreased and homogenized detrital stoichiometry) and increased C loss from stream ecosystems.

Cumulative Effects of Nutrient Enrichment and Elevated Temperature Compromise the Early Life History Stages of the Coral Acropora tenuis.

Inshore coral reefs are experiencing the combined pressures of excess nutrient availability associated with coastal activities and warming seawater temperatures. Both pressures are known to have detrimental effects on the early life history stages of hard corals, but studies of their combined effects on early demographic stages are lacking. We conducted a series of experiments to test the combined effects of nutrient enrichment (three levels) and elevated seawater temperature (up to five levels) on early life history stages of the inshore coral Acropora tenuis, a common species in the Indo-Pacific and Red Sea. Gamete fertilization, larval survivorship and larval settlement were all significantly reduced as temperature increased, but only fertilization was further affected by simultaneous nutrient enrichment. Combined high temperatures and nutrient enrichment affected fertilization in an additive manner, whereas embryo abnormalities increased synergistically. Higher than normal temperatures (32°C) increased coral juvenile growth rates 1.6-fold, but mortality also increased by 50%. The co-occurrence of nutrient enrichment with high temperatures reduced juvenile mortality to 36%, ameliorating temperature stress (antagonistic interaction). Overall, the types of effect (additive vs synergistic or antagonistic) and their magnitude varied among life stages. Gamete and embryo stages were more affected by temperature stress and, in some cases, also by nutrient enrichment than juveniles. The data suggest that coastal runoff events might exacerbate the impacts of warming temperatures on fertilization if these events co-occur during corals spawning. The cumulative impacts of simultaneous exposure to nutrient enrichment and elevated temperatures over all early life history stages increases the likelihood for failure of larval supply and recruitment for this coral species. Our results suggest that improving the water quality of river discharges into coastal areas might help to enhance the thermal tolerances of early life history stages in this common coral species.

Interactive effects of three pervasive marine stressors in a post-disturbance coral reef

Ecosystems are commonly affected by natural, episodic disturbances that can abruptly and drastically alter communities. Although it has been shown that resilient ecosystems can eventually recover to pre-disturbed states, the extent to which communities in early stages of recovery could be affected by multiple anthropogenic stressors is poorly understood. Pervasive and rising anthropogenic stressors in coastal marine systems that could interactively affect the recovery of these systems following natural disturbances include high sedimentation, nutrient enrichment, and overfishing. Using a 6-month field experiment, we examined the effects of all combinations of these three stressors on key functional groups in the benthic community growing on simulated, post-disturbance reef patches within a system recovering from large-scale natural disturbances (corallivorous seastar outbreak and cyclone). Our study revealed that sedimentation, nutrient enrichment, and overfishing (simulated using exclusion cages) interactively affected coral survival and algal growth, with taxon-specific effects at multiple scales. First, our treatments affected corals and algae differently, with sedimentation being more detrimental to macroalgal growth but less detrimental to coral (Porites rus) survival in caged plots, driving significant interactions between sedimentation and caging for both taxa. We also observed distinct responses between coral species and between algal functional groups, with the most extensive responses from algal turf biomass, for which sedimentation suppressed the synergistic (positive) combined effect of nutrient enrichment and caging. Our findings suggest that different combinations of ubiquitous anthropogenic stressors, related to either sea- or land-based activities, interactively influence community recovery from disturbance and may alter species compositions in the resulting community. Our findings further suggest that anthropogenic stressors could promote further degradation of coral reefs following natural disturbances by inhibiting recovery to coral-dominated states that provide vital ecosystem services to coastal populations worldwide.

Long‐term fertilization alters the relative importance of nitrate reduction pathways in salt marsh sediments

AbstractSalt marshes provide numerous valuable ecological services. In particular, nitrogen (N) removal in salt marsh sediments alleviates N loading to the coastal ocean. N removal reduces the threat of eutrophication caused by increased N inputs from anthropogenic sources. It is unclear, however, whether chronic nutrient overenrichment alters the capacity of salt marshes to remove anthropogenic N. To assess the effect of nutrient enrichment on N cycling in salt marsh sediments, we examined important N cycle pathways in experimental fertilization plots in a New England salt marsh. We determined rates of nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) using sediment slurry incubations with 15N labeled ammonium or nitrate tracers under oxic headspace (20% oxygen/80% helium). Nitrification and denitrification rates were more than tenfold higher in fertilized plots compared to control plots. By contrast, DNRA, which retains N in the system, was high in control plots but not detected in fertilized plots. The relative contribution of DNRA to total nitrate reduction largely depends on the carbon/nitrate ratio in the sediment. These results suggest that long‐term fertilization shifts N cycling in salt marsh sediments from predominantly retention to removal.

Effect of nutrient enrichment on the source and composition of sediment organic carbon in tropical seagrass beds in the South China Sea

To assess the effect of nutrient enrichment on the source and composition of sediment organic carbon (SOC) beneath Thalassia hemprichii and Enhalus acoroides in tropical seagrass beds, Xincun Bay, South China Sea, intertidal sediment, primary producers, and seawater samples were collected. No significant differences on sediment δ13C, SOC, and microbial biomass carbon (MBC) were observed between T. hemprichii and E. acoroides. SOC was mainly of autochthonous origin, while the contribution of seagrass to SOC was less than that of suspended particulate organic matter, macroalgae and epiphytes. High nutrient concentrations contributed substantially to SOC of seagrass, macroalgae, and epiphytes. The SOC, MBC, and MBC/SOC ratio in the nearest transect to fish farming were the highest. This suggested a more labile composition of SOC and shorter turnover times in higher nutrient regions. Therefore, the research indicates that nutrient enrichment could enhance plant-derived contributions to SOC and microbial use efficiency.

Succession in an intertidal benthic community affected by untreated sewage effluent: A case of study in the SW Atlantic shore

A study of benthic succession related with sewage pollution was conducted in a warm-temperate coastal area of Mar del Plata city, Argentina. The effect of nutrient enrichment and starting period of the succession was tested after cleared space in the intertidal rocky shore benthic community. The time of recovery after a disturbance in enriched sites was considerably lower (20–29weeks) than in non-enriched sites based on the diatom Berkeleya sp. and the polychaete Boccardia proboscidea developed in the early succession stage, and Brachidontes rodriguezii or B. proboscidea occurring with Polysiphonia sp., Petalonia fascia and Ulva spp. developed in the late succession stage of the enriched sites. The very low time at which the community recovered in the enriched environments turned out to be one of the assets of the present study and this parameter can be used as a quick indicator of sewage pollution in the area.

Light availability regulates the response of algae and heterotrophic bacteria to elevated nutrient levels and warming in a northern boreal peatland

Summary Light attenuation associated with elevated levels of dissolved organic matter (i.e. browning) is likely to affect the response of benthic biofilms to nutrient enrichment and warmer temperatures expected for northern aquatic ecosystems with climate change. To examine how these factors will interact to regulate the association between algae and heterotrophic bacteria in northern peatlands, we manipulated light (100%, 60%, 30%, 20% and 5% of ambient), nutrients (unenriched, enriched) and temperature (ambient, warming), in a full factorial design by shading nutrient diffusing substrates inside mesocosms with or without warming (3 °C) in an Alaskan fen. After 12 days of experimental conditions, there was no effect of light or temperature on the abundance of algae or bacteria in the absence of nutrient enrichment. Enrichment with a combination of nitrogen and phosphorus significantly increased the abundance of algae and heterotrophic bacteria at ambient light levels (418 μmol m−2 s−1), and warming significantly enhanced the positive effects of nutrients on algal accrual. Although warming enhanced the effects of nutrient enrichment on bacterial growth, the magnitude of the effect was not statistically significant compared to nutrient enrichment alone. The positive influence of nutrient enrichment and the synergistic effects of nutrients and warming on algae and bacteria were lost at ≤30% ambient light (below 125 μmol m−2 s−1). We conducted a separate laboratory incubation using recirculating water baths maintained at either ambient or elevated water temperature (warming of 5 °C) to evaluate the effects of warming on the use of algal exudates by heterotrophic bacteria. During the 16‐day assay, the abundance of heterotrophic bacteria increased rapidly in the presence of a common solution of algal exudates and the rate of exudate assimilation by bacteria was significantly enhanced by warming compared with ambient temperature. The positive association between light availability and heterotrophic bacteria, coupled with accelerated bacterial growth in the presence of algal exudates, support bacterial dependence on algal subsidies in the presence of recalcitrant organic substrates in this northern peatland. Within a predictive context, our results demonstrate that greater light attenuation associated with browning of surface water may reduce the positive influence of warmer temperatures and elevated nutrient levels on the microbial loop by reducing algal production on submerged surfaces in northern peatlands.

Interactive effects of increasing temperature and nutrient loading on the habitat-forming rockweed Ascophyllum nodosum

Perennial seaweeds are dominant primary producers and foundation species along rocky shores, providing essential ecosystem functions and services. Although increasingly affected by various anthropogenic activities, the cumulative effects of multiple stressors are little known. We tested the interactive effects of nutrient enrichment and increased water temperatures on growth, nitrogen retention and carbon storage in juvenile Ascophyllum nodosum from Nova Scotia, Canada (44° 29.9′ N, 63° 31.7′ W) using a multi-factorial laboratory experiment. Temperature strongly affected growth, significantly reducing weight and length gain from 16°C to 20°C and 24°C. Medium nutrient enrichment enhanced while high enrichment slowed rockweed growth at lower temperatures, yet these effects disappeared with warming. Nitrogen retention in rockweed tissue significantly increased with nutrient enrichment and decreased with warming, whereas carbon storage remained unaffected. These individual and interactive effects of nutrient loading and climate warming may alter the structure and function of rockweed habitats with potentially far-reaching ecological and economic consequences.

The response of epiphytic bacteria on Vallisneria natans (Lour.) Hara (Hydrocharitaceae) to increasing nutrient loadings.

To investigate the effects of water column nutrient loading on epiphytic bacteria, we determined the abundance and community composition of epiphytic bacteria on the submerged macrophyte Vallisneria natans (Lour.) Hara during the growth season (June-October) under four different nutrient concentrations (nitrogen (N)-phosphorus (P) in mg L(-1) : 0.5-0.05, 1.0-0.1, 5.0-0.5, 10.0-1.0; hereafter NP-1, NP-2, NP-3, NP-4, respectively), using epifluorescence microscopy method and terminal restriction fragment length polymorphism (T-RFLP) analysis, respectively. Relative to low nutrient conditions (NP-1), there was no significant effect on the epiphytic bacterial community, and even a decrease in the number of epiphytic bacteria, which linked to the well growth status of host macrophytes at moderate nutrient conditions (NP-2). However, further nutrient enrichment induced significant increase in the abundance of epiphytic bacteria, and marked changes in the community structures of epiphytic bacteria. Furthermore, at high nutrient conditions, epiphytic bacterial communities varied widely temporally, and were not stable compared with those at the lower nutrient conditions. These results indicated that the effects of nutrient enrichment on epiphytic bacteria were nonlinear and dependent on the nutrient concentrations in the water.

Complex interactions between nutrient enrichment and zooplankton in regulating estuarine phytoplankton assemblages: Microcosm experiments informed by an environmental dataset

The interactive effects of nutrients and zooplankton grazing on phytoplankton assemblage composition and bloom formation are not well understood, especially in coastal ecosystems. Therefore, the objective of this study was to evaluate phytoplankton assemblage responses to changing environmental conditions in Raritan Bay, an estuary between the states of New York and New Jersey with a long history of cultural eutrophication and harmful algal blooms (HABs). Environmental monitoring of water quality and plankton species composition (monthly data collected from April 2010–April 2013), multivariate ordination techniques, and microcosm experiments were integrated to achieve this objective. Multivariate analysis of the monitoring dataset led to the generation of a number of hypotheses regarding phytoplankton composition, individual nuisance species, environmental factors, and zooplankton composition. The field observations were then supplemented with a series of controlled microcosm experiments designed to test those hypotheses. In particular, the effects and interaction of varying mesozooplankton abundance (sieved, unsieved and enriched) and enrichment with two nutrients (nitrate and dissolved iron) on spring and summer phytoplankton assemblages were evaluated. The environmental monitoring data and the results of the microcosm experiments both indicate that Si:N ratios are important factors governing phytoplankton dynamics in Raritan Bay and that effects of nutrient enrichment on phytoplankton species composition are magnified when mesozooplankton abundance is low. The microcosm experiments, however, led to unexpected results regarding the influence of iron on phytoplankton assemblages. Iron did not emerge as one of the environmental parameters related to spring and summer phytoplankton species composition in the multivariate analysis of field data. Yet, enrichment with iron in microcosms resulted in significant increases in diatoms, and enrichment with both iron and nitrate resulted in significant increases in dinoflagellates and HAB taxa, including Heterocapsa triquetra (Ehrenberg) Stein and Dinophysis spp. This finding suggests that small pulses of iron, in combination with low Si:N ratios (<1) and low mesozooplankton abundance, would shift phytoplankton assemblages in Raritan Bay toward greater dinoflagellate dominance.

Biochemical quality of basal resources in a forested stream: effects of nutrient enrichment

We studied biochemical changes in biofilm and suspended particulate and dissolved organic matter (OM) during the leaf emergence period (March–May 2008) in a forested headwater stream in response to a long-term (4 years, 2004–2008) experimental nutrient enrichment study. This study compared results from one reach upstream of the enrichment point and one reach downstream using moderate nutrient concentrations (nitrogen, N, from 388 to 765 μg L−1 and phosphorus, P, from 10 to 30 μg L−1, resulting in N:P ratios of 85–56). During the spring of 2008, we analysed the chlorophyll content, elemental composition (carbon, C, and N), bacterial density, and extracellular enzyme activities along with their biochemical composition (amino acids, fatty acids and sterols) on biofilm and OM. Nutrients caused changes in the biochemical composition of the biofilm, while changes in the OM were subtle. The C:N ratio of the biofilm decreased with nutrient enrichment likely due to the increase in protein (non-essential amino acids). The polysaccharide and total and essential fatty acid contents were higher when nutrient enrichment coincided with greater light availability. The peptidase extracellular activity was higher in the fertilised reach at early spring, while phosphatase activity decreased at late spring. The suspended and dissolved OM composition did not change due to the nutrient addition, likely due to the lower water residence time in the reach. Headwater systems are highly dynamic, and the biochemical composition of the biofilm changed in response to changes in nutrients but also to light in this study. These changes, although moderate, could influence higher trophic levels through modifications in their diet. This experiment exemplifies how small land use shifts may affect headwater streams.

Autochthony, allochthony and the role of consumers in influencing the sensitivity of aquatic systems to nutrient enrichment

Primary consumers may mitigate or exacerbate the effects of nutrient enrichment by grazing on plant matter and recycling nutrients. Few studies have quantified these effects for a suite of consumers and in the context of other processes regulating plant standing crop and nutrient supply. We quantified the abundance, feeding and diet of zooplankton, benthic filter-feeders, and planktivorous and detritivorous fish in the James River Estuary and found that consumer-mediated fluxes of CHLa and N were small in comparison to other fluxes regulating phytoplankton abundance (production, respiration, advection) and N availability (external inputs, internal recycling). Chlorophyll-a ingestion by consumers was equivalent to 15% of daily phytoplankton production and N recycling by consumers corresponded to 29% of phytoplankton N demand. The bulk of phytoplankton production (74%) was lost to autotrophic and heterotrophic respiration. Recycling of N contained in autochthonous and allochthonous organic matter was sufficient to meet 100% of phytoplankton N demand with external inputs corresponding to 39% of phytoplankton N demand. Experiments using 2000L outdoor mesocosms with natural plankton communities showed that the presence of consumers (shad) resulted in higher N availability, elevated CHLa and reduced macrozooplankton abundance. Overall, our study showed that the direct effect of consumers on fluxes of CHLa and N was small as the bulk of phytoplankton biomass and N passed to microbial decomposers. However selective grazing by planktivorous fishes reduced zooplankton densities resulting in greater phytoplankton yield at lower nutrient concentrations.

Widespread natural intraspecific variation in tissue stoichiometry of two freshwater molluscs: Effect of nutrient enrichment

A central premise of ecological stoichiometry is that consumers maintain relatively fixed elemental composition in their bodies, a process known as elemental homoeostasis. Although nutrient enrichment is a ubiquitous problem facing many freshwater lakes around the world, intraspecific variation in elemental composition of freshwater invertebrates and its relationship with nutrient loading have not been well addressed. Here, we examined carbon:nitrogen:phosphorus (C:N:P) stoichiometry of two widely distributed molluscs, Corbicula fluminea and Bellamya aeruginosa, from several subtropical shallow lakes across a nutrient gradient. Our results showed that these two species exhibited substantial natural intraspecific variation in tissue stoichiometry which can reach or even exceed the values among different freshwater taxa investigated before. Our results suggest that tissue P content presents the greatest variations, followed by N content, and lowest in C content. Tissue P content ranged about three-fold (0.56%–1.65%) and five-fold (0.41%–2.28%) for B. aeruginosa and C. fluminea, respectively. Correspondingly, N content ranged from 5.16% to 12.06% and from 6.47 to 11.36%, respectively. Tissue %P, C:P and N:P ratios were strongly correlated with PO43−-P and/or chlorophyll-a in the water column. Tissue N and P contents of B. aeruginosa and P content of C. fluminea increased with increasing lake trophic levels (mesotrophic to eutrophic to hypertrophic). These results suggest that the two molluscs can adjust their tissue stoichiometry in relation to nutrient enrichment. Relaxing the assumption of strict homeostasis may help them cope with potential stoichiometric constraints. The results provide additional clues to why these two species are successful invaders and widely distributed.