Water Column Nutrient Concentrations Research Articles

Long-term patterns and trends in water column biogeochemistry in a changing environment

Water column nutrient concentrations and autotrophy in oligotrophic ecosystems are sensitive to eutrophication and other long-term environmental changes and disturbances. Disturbance can be defined as an event or process that changes the structure and response of an ecosystem to other environmental drivers. The role disturbance plays in regulating ecosystem functions is challenging because the effect of the disturbance can vary in magnitude, duration, and extent spatially and temporally. We measured changes in total nitrogen (TN), dissolved inorganic nutrient (DIN), total phosphorus (TP), soluble reactive phosphorus (SRP), total organic carbon (TOC), and chlorophyll-a (Chl-a) concentrations throughout the Everglades, Florida Bay, and the Florida Keys. This region has been subjected to a variety of natural and anthropogenic disturbances including tropical storms, fires, eutrophication, and rapid increases in water levels from sea-level rise and freshwater restoration. We hypothesized that the rate of change in water quality would be greatest in the coastal ecotone where disturbance frequencies and marine P concentrations are highest, and in freshwater marshes closest to hydrologic changes from restoration. We applied trend analyses on multi-decadal data (1996–2019) collected from 461 locations distributed from inland freshwater Everglades (ridge and slough) to outer marine reefs along the Florida Keys, USA. Total Organic Carbon decreased throughout the study area and was the only parameter with a systematic trend throughout the study area. All other parameters had spatially heterogeneous patterns in long-term trends. Results indicate more variability across a large spatial and temporal extent associated with changes in biogeochemical indicators and water quality conditions. Chemical and biological changes in oligotrophic ecosystems are important indicators of environmental change, and our regional ridge-to-reef assessment revealed ecosystem-specific responses to both long-term environmental changes and disturbance legacies.

Nutrient fluxes, budgets and net ecosystem metabolism in a Brazilian coastal system under drought conditions

Integrated dissolved inorganic nutrient fluxes (nitrite, nitrate, ammonium, phosphate and silicate) between Patos Lagoon estuary (PLE), Brazil, and the inner continental shelf of the southwest Atlantic Ocean, were estimated from averaged water column nutrient concentrations and water fluxes (method 1 – M1). Daily surveys were carried out for 30 days, spanning from Feb-22 to Mar-23 of 2021 during late austral summer drought, at two stations — one next to the estuary mouth, the other in the inner estuary, at the interface between the estuary main channel and a shallow embayment called the Arraial bight. On average, PLE delivered dissolved inorganic phosphorus (DIP), nitrogen (DIN) and silicate outfluxes to the adjacent continental shelf at rates of 22.4 ± 596.1, 89.8 ± 207.3 and 1715.9 ± 3283.2 × 103 mmol m−2 d−1, respectively. Budgets for DIP and DIN were computed by the LOICZ box model approach (method 2 – M2) considering Patos Lagoon divided into its limnic (fresh, mean salinity 2 psu) and estuarine (brackish, mean salinity 18.5 psu) regions. The residual fluxes for DIP and DIN from PLE to the inner continental shelf were similar to those of M1, but the complete budget (M2) revealed that PLE also imported DIP, at a positive exchange rate between PLE and coastal waters. There is evidence that DIP is supplied by Plata Plume Water. Net ecosystem metabolism (NEM) and N-fixation – denitrification were assessed through the LOICZ budgets for DIP and DIN, respectively. Denitrification prevailed in both limnic and estuarine regions. Estimated NEM indicated weak autotrophy in the limnic region, and net heterotrophy in the estuarine region. The net heterotrophy in the estuarine region was mainly attributed to nitrogen limitation due to low allochthonous supply by river discharge, and N-loss by denitrification during austral summer drought conditions.

Growth response to nitrate enrichment helps facilitate success of an alien Potamogeton in New Zealand streams

Motivated by stream ecosystem degradation by eutrophication, we mimicked slow flowing lowland stream conditions with a novel experimental setup to further our understanding of aquatic plant responses to increases in nitrate and light. We conducted a mesocosm growth experiment of two species from the genus Potamogeton: P. crispus (alien) and P. ochreatus (native), grown at four nitrate and four light levels. We hypothesised that (i) internal nutrient status of the plants would scale with water column nutrient concentration, and that (ii) plant performance would reflect the nutrient status of the plant. Furthermore, we hypothesised that (iii) a low irradiance level would negate the effects of an increased nitrate level. In relation to (ii) we hypothesised that (iv) the traits of the alien species would enable it to outperform the native species where both the availability of light and nutrient resources was high. Internal tissue N content was broadly similar in the two higher (>250 μg NO3− L−1) and the two lower nutrient treatments (<20 μg NO3− L−1) in both species and plants were therefore collapsed into high and low N-groups. High-N individuals had higher growth rates than low-N ones regardless of species or light treatment and plants had reduced growth rates at the lowest light treatment, however this response was less evident for P. crispus. The highest growth rate was found at the high-N individuals of P. crispus at the highest light treatment, and correspondingly, in this treatment this species exhibited an increase in branching degree and lateral spread from the low-N plants. As P. crispus spreads by fragmentation, our results show it to be a highly effective competitor in anthropogenically impacted areas compared to its native counterpart. Our study exemplifies how light can influence eutrophication responses of plants and how both need to be accounted for in management decisions.

River metabolic fingerprints and regimes reveal ecosystem responses to enhanced wastewater treatment.

Although many studies have examined how improvements in wastewater treatment impact river nutrient concentrations and loads, there has been much less focus on measuring river metabolism to evaluate the wider aquatic ecosystem benefits of reducing nutrient inputs to rivers. The objectives of this study were to evaluate the effects of enhanced wastewater treatment (nitrification) on river metabolism in the Grand River, Canada's largest river draining into Lake Erie. Metabolic fingerprints and regimes (calculated from high-frequency dissolved oxygen [DO] measurements) were used to visualize whole-river ecosystem functional responses to these wastewater treatment upgrades. There was a 60% reduction in ecosystem respiration during summer, in response to reductions in effluent total ammonia inputs, causing a shift from net heterotrophy to net autotrophy, and contraction of river metabolic fingerprints. This resulted in major improvements in summer DO concentrations, with reductions in the percentage of days during summer that DO minima fell below water-quality guidelines for protection of aquatic early life stages, from 88% to ≤16%. The results also point to potential cascading impacts on coupled phosphorus and nitrogen cycles, which may generate further improvements in river water quality. During the summer, high rates of river metabolism and nutrient retention may result in measured water-column nutrient concentrations potentially underestimating nutrient pressures. This study also demonstrates the value of combining river metabolism with nutrient monitoring for a more holistic understanding of the role of nutrients in river ecosystem health and function.

The influence of parasitism on producers and nutrients in mesocosm ecosystems

Pathogens and parasites are increasingly recognized as important components within host populations, communities, and ecosystems. Both density-mediated and trait-mediated impacts of parasites on ecosystems are known and likely operate together to influence ecosystem processes. Despite the assertion that trait-mediated impacts of parasites are pervasive, empirical evidence of these effects is lacking. Our aim is to fill this gap and test whether parasitism can influence ecosystem processes within controlled mesocosm ecosystems. In the host-parasite (snail-trematode) system used, parasites form cysts in second intermediate host tissue and cause minimal direct mortality (minimizing density-mediated parasite impacts). We created mesocosms across a gradient of parasitism and measured water column nutrient concentrations, producer biomass, and invertebrate community composition. We demonstrate that trematode parasitism is correlated with an increase in periphyton dry mass and percent ash-free dry mass. Additionally, water column carbon and phosphorus concentrations were influenced by producers but not parasites. We demonstrate that parasites in the metacercarial stage have limited impact as “ecosystem engineers”, but some data suggest parasites may have a subtle influence on ecosystem processes.

Water column nutrient concentrations are related to excretion by benthic invertebrates in Lake Taihu, China

Internal release of nutrients is an important contributor to the nutrient dynamics in shallow eutrophic lakes. Zoobenthic organisms may contribute to this release by excreting nutrients to the overlaying water. Based on experiments and using results from previous experimental studies as well as field monitoring density data from 2007 to 2017, we calculated the annual and seasonal nutrient excretions of the two most common macroinvertebrates (Corbicula fluminea and Limnodrilus hoffmeisteri) in Lake Taihu, China. We compared these rates with the concentrations of NH4–N, total nitrogen (TN), PO4–P and total phosphorus (TP) in the lake water as well as with previous results of release rates from undisturbed sediments collected in the lake. The spatial distribution of nutrient excretion by the two invertebrate species varied markedly among sites and years. Regression analyses revealed significant relationships between total nutrient excretions by these two species and the concentrations of NH4–N, TN, PO4–P and TP in the lake, but with seasonal differences. The relationship was overall strongest in winter, followed by spring, and weakest in summer and autumn. The flux of NH4–N and PO4–P released by the two macroinvertebrate species were equivalent to as much as 50% and 66%, respectively, of the sediment release recorded in lab experiments under undisturbed conditions; however, the percentages would be somewhat lower under field conditions where the sediment is subjected to frequent wind-induced resuspension and fish disturbance, enhancing the release rates. The release declined during the study period due to a reduction in the density of macroinvertebrates, perhaps indicating increasing stocking of fish since 2007. Our results indicate that benthic invertebrates are important contributor to the internal loading in shallow eutrophic lakes.

Distinguishing nutrient-depleting effects of scallop farming from natural variabilities in an offshore sea ranch

While bivalve farming is well recognized in terms of modifying the biogeochemical cycle through filter-feeding and biodeposition, its impacts on water column nutrient concentrations in various ecosystems may vary from depletion to addition. Hypothesizing that the variability arises from overlap of farming-associated impacts and pelagic processes, nutrient concentrations and phytoplankton biomass were investigated monthly in an offshore sea ranch, to detect the real impacts of bottom-seeding scallop farming. Nutrient concentrations in water column average increased from July to December following successive surface freshening and disappearance of thermal stratification. Abrupt decrease was observed in March, along with increase in chlorophyll a concentration. The silicate concentration decreased below 2 μM in all depth layers in the mariculture area in March and April, whereas silicate limitation in open waters was commonly observed in April. Compared to open waters, inorganic nitrogen and phosphate concentrations were significantly lower in the mariculture area on an annual cycle, whereas silicate concentration was lower in spring and higher in summer. Our results indicate that scallop farming in frequently refreshed waters can act as nutrient sink through top-down effects. Without exogenous supply, it can also introduce the limitation of nutrient (silicate) with low regeneration rate and stimulate shift in the phytoplankton community structure.

River nutrient water and sediment measurements inform on nutrient retention, with implications for eutrophication

The consideration of nutrients in pollution dynamics is important for environmental management and conservation. Developing countries are yet to appreciate the aquatic ecosystem pollution impacts on their economies and as such, information on water pollution dynamics is limited. This study assessed the spatio-temporal dynamics of nutrient loading and retention in stream water and sediments in the Bloukrans River system, Eastern Cape province, South Africa over the course of the wet and dry season. Sediment and water samples were analysed for total phosphorus (TP) and nitrogen (TN) concentrations, and were used in combination with river flow discharge, to determine nutrient loads. The study results highlight that river discharge plays a significant role in temporal differences in sediment and water column nutrient concentrations. The mean sediment nutrient concentration was high for the dry season, with high values being observed for the urban river system. Nutrient loads were high above the sewage treatment works outflow (i.e. urban sites), as such, a decreasing trend was observed with increasing distance from the urban environment. Nutrient loads were generally high for the dry season in comparison to the wet season indicating organic matter retention (i.e. accumulation from burst sewage pipes) most likely due to low flows. While it was evident that the ageing wastewater infrastructure contributed to the observed state of the Bloukrans River, the high natural nutrient retention capacity seemed to mitigate eutrophication of downstream aquatic ecosystems. As such, the nutrient retention capacity and management of the system is central to the entire Bloukrans River catchment management practices. Therefore, the study contributes to our understanding of water and sediment nutrient pollution dynamics in an arid temperate river landscape where vast spatio-temporal differences in base flow characterise the riverscape.

Physical-biogeochemical interactions and potential effects on phytoplankton and Ulva prolifera in the coastal waters off Qingdao (Yellow Sea, China)

In recent years, the spectacular massive green tide of Ulva prolifera has become a recurrent phenomenon appearing every summer in the coastal waters off Qingdao (Yellow Sea, China), attracting the attention of scientists and local government. Based on multidisciplinary data collected during summer and winter, this study focuses on the hydrological characteristics and regional biogeochemical processes in coastal waters off Qingdao. The results show that the boundary of the Yellow Sea Cold Water Mass (YSCWM) can reach the Qingdao coastal region in summer and is locally raised to the upper layers to form coastal upwelling beyond tidal mixing and favorable wind. The regional summer upwelling off the Qingdao coast effectively enriches the nutrient concentrations in the upper water column and thus promotes growth of phytoplankton but reduces the dissolved oxygen (DO) concentration and pH value in the bottom. The regional summer upwelling off Qingdao coast may facilitate the growth and regional blooming of the U. prolifera that migrate to this region with the southerly wind. Additionally, the effects of the front on the aggregation of U. prolifera may be significant. In winter, the Yellow Sea Warm Current (YSWC) extends and spreads along the offshore region off the Subei Shoal towards the Qingdao coastal sea. This tongue-shaped warm water meets the cold coastal water off Qingdao, which leads to the formation of a physical front. As a consequence, remarkable fronts of nutrient and chlorophyll a (Chl a) also form between the shoreward warm water and the cold coastal water. This study increases the understanding of the interactions between the regional physical, chemical, and biological processes off the Qingdao coast.

Spatio-temporal dynamics in the dissolved nutrient waste plume from Norwegian salmon cage aquaculture

AEI Aquaculture Environment Interactions Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections AEI 10:385-399 (2018) - DOI: https://doi.org/10.3354/aei00276 Spatio-temporal dynamics in the dissolved nutrient waste plume from Norwegian salmon cage aquaculture H. M. Jansen1,2,*, O. J. Broch3, R. Bannister1, P. Cranford4, A. Handå3, V. Husa1, Z. Jiang5, T. Strohmeier1, Ø. Strand1 1Institute of Marine Research, Nordnesgaten 50, 5005 Bergen, Norway 2Wageningen University and Research, Wageningen Marine Research (WMR), PO Box 77, 4400 AB Yerseke, The Netherlands 3SINTEF Ocean, Postboks 4762 Torgarden, 7465 Trondheim, Norway 4Fisheries and Oceans Canada, St. Andrews Biological Station, 125 Marine Science Dr., Saint Andrews, New Brunswick E5B 0E4, Canada 5Yellow Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, 106 Nanjing Road, Qingdao, Shandong Province, PR China *Corresponding author: henrice.jansen@wur.nl ABSTRACT: The aim of this study was to define the waste plume dynamics around a salmon farm in Norway. Systematic water sampling and numerical modeling were implemented to define nutrient concentrations in the upper water column at long-term (seasonal) and short-term (between and within days) time scales. Nutrient enhancement was observed for ammonium only, while the concentrations of orthophosphate and organic wastes were never higher than the background values. The spatial magnitude of cage effluent dispersion was limited. Empirical results detected enhanced concentrations up to 100 m down-current of the farm when fish biomass was high. Model results showed that the zone of influence could occasionally reach to >1000 m. In the first year of production, when fish biomass was low, no enhancement was detected, and in April and September of the following year, average ammonium concentrations were respectively 0.2 and 0.8 µM above the background concentrations. Taking the ambient seasonal variability into account, this resulted in 1.6 times higher concentrations for both sampling months. The measured short-term temporal variability in nutrient concentrations near the cages varied up to 2 times from day to day and were 3.5 times higher in the evening compared to the morning. As seasonal investigations were performed in the morning, maximum enhancement was likely underestimated. The rapid decrease in nutrient concentrations with increasing distance from the cages suggests that the farm studied here is currently not causing significant degradation of surface water quality. Results of this study contribute to evaluating the potential for ecological mitigation of waste nutrients and provide directions for design of optimized integrated multi-trophic aquaculture facilities. KEY WORDS: Atlantic salmon · Farm scale · Nutrients · Nitrogen · Integrated multi-trophic aquaculture · IMTA · Pelagic · Environmental impact Full text in pdf format Supplementary material PreviousNextCite this article as: Jansen HM, Broch OJ, Bannister R, Cranford P and others (2018) Spatio-temporal dynamics in the dissolved nutrient waste plume from Norwegian salmon cage aquaculture. Aquacult Environ Interact 10:385-399. https://doi.org/10.3354/aei00276 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in AEI Vol. 10. Online publication date: September 27, 2018 Print ISSN: 1869-215X; Online ISSN: 1869-7534 Copyright © 2018 Inter-Research.

Nitrate amendment reduces biofilm biomass and shifts microbial communities in remote, oligotrophic ponds

AbstractHumans have increased the amount of reactive N available in the environment by over an order of magnitude since the industrial revolution. Most studies have been conducted in ecosystems with pervasive anthropogenic nutrient inputs, so little is understood about how naive biofilm communities respond to elevated nutrients. Our nutrient-diffusing substrate (NDS) experiments, which were conducted in Alaskan freshwater ponds with very little anthropogenic nutrient inputs, suggest that P limits biofilm photoautotrophs. However, despite low water-column nutrient concentrations, overall biofilm biomass was not enhanced by the addition of N or P. Rather, we observed an ~60% biomass reduction with NO3– amendment in 15 oligotrophic ponds across 2 y. This widespread biomass reduction was accompanied by changes in microbial communities, but these trends were not observed with NH4+ or P amendment. Nonamended communities (i.e., no nutrient amendment other than lysogeny broth agar) were characterized by anaerobic...

Interactive Effects of Physical and Biogeochemical Feedback Processes in a Large Submersed Plant Bed

Submersed plants are sensitive to nutrient loading because excess algal growth creates light-limiting conditions. However, submersed plant beds can also modify nutrient cycling through feedback loops whereby algal growth is limited and plant growth is enhanced. Whereas most studies on the effect of submersed aquatic vegetation (SAV) beds on nutrient cycling concentrate on either biogeochemical or physical controlling mechanisms, we use a holistic approach that analyzes how these processes interact. We measured a suite of physical and biological processes in a large SAV bed and developed a simple, 1-dimensional reactive transport model to investigate the mechanisms underlying SAV bed effects on nutrient cycling. We observed lower water column concentrations of dissolved inorganic nitrogen and phosphorus (DIN and DIP) inside relative to outside the SAV bed during the summer. Sediment denitrification (mean N2-N flux in August = 46 μmol m−2 h−1) and plant nutrient assimilation (August rates =385 μmol N and 25 μmol P m−2 h−1) were mechanisms of nutrient removal. We also found that the physical structure of the bed decreased advection and tidal dispersion, resulting in increased water residence time that enhanced biologically mediated nutrient loss. These processes create conditions that enable SAV to outcompete other primary producers, as water column nutrient concentrations were low enough to limit algal growth and associated light attenuation, while sediment pore water concentrations were sufficient to satisfy SAV nutrient demand. These findings suggest that interactions between physical and biological feedback processes in SAV beds can play a key role in structuring shallow aquatic ecosystems.

Short-Term Nitrogen and Phosphorus Release during the Disturbance of Surface Sediments: A Case Study in an Urbanised Estuarine System (Gold Coast Broadwater, Australia)

Understanding the effects of sediment disturbances on nutrient loadings is important for the management of estuarine settings. This study investigated the initial influence of sediment disturbance on water column nutrient concentrations in a shallow estuarine setting within the Gold Coast Broadwater, using a laboratory-based approach. Undisturbed sediment cores (200 mm Ø × 330 mm length, plexiglass) were incubated before and after being subjected to a disturbance event, to investigate the effect on the immediate and subsequent short-term water column nutrient concentrations. Sediment NH4+bio and PO43−bio concentrations ranged from 150 to 478 and 1.50 to 8.56 nmol g−1 dry wt, respectively. Water column NH4+ concentrations underwent the greatest increase (&gt;1000% or approx. 14 times greater) immediately following disturbance, with mean effluxes increasing by &gt;300%. Thereafter, water column NH4+ concentrations and efflux rates declined to near initial pre-disturbance concentrations. Water column NH4+ concentrations accounted for 0.58%–5.50% of the depth-integrated sediment NH4+bio concentration, indicating mobilization of the sediment bound exchangeable NH4+. The observed changes in PO43− concentrations and fluxes were much lower in comparison to those observed for N-species. Following disturbance, increases in the water column PO43− concentration accounted for 7.16%–8.22% depth-integrated sediment bioavailable PO43− at +1 and +2 hours, and 5.65% at +7 hours, respectively. These results provide important insight into the potential implications of disturbance events, such as vessel activities and dredging operations, within the case study region, providing information for potential management options and relevant water quality concerns.

Potential for Local Fertilization: A Benthocosm Test of Long-Term and Short-Term Effects of Mussel Excretion on the Plankton.

Mussel aquaculture has expanded worldwide and it is important to assess its impact on the water column and the planktonic food web to determine the sustainability of farming practices. Mussel farming may affect the planktonic food web indirectly by excreting bioavailable nutrients in the water column (a short-term effect) or by increasing nutrient effluxes from biodeposit-enriched sediments (a long-term effect). We tested both of these indirect effects in a lagoon by using plankton-enclosing benthocosms that were placed on the bottom of a shallow lagoon either inside of a mussel farm or at reference sites with no history of aquaculture. At each site, half of the benthocosms were enriched with seawater that had held mussels (excretion treatment), the other half received non-enriched seawater as a control treatment. We monitored nutrients ([PO43-] and [NH4+]), dissolved oxygen and plankton components (bacteria, the phytoplankton and the zooplankton) over 5 days. We found a significant relationship between long-term accumulation of mussel biodeposits in sediments, water-column nutrient concentrations and plankton growth. Effects of mussel excretion were not detected, too weak to be significant given the spatial and temporal variability observed in the lagoon. Effects of mussels on the water column are thus likely to be coupled to benthic processes in such semi-enclosed water bodies.

Impact of water-level fluctuations on cyanobacterial blooms: options for management

Climate change can promote harmful cyanobacteria blooms in eutrophic waters through increased droughts or flooding. In this paper, we explore how water-level fluctuations affect the occurrence of cyanobacterial blooms, and based on the observations from case studies, we discuss the options and pitfalls to use water-level fluctuations for lake and reservoir management. A drawdown in summer causes an increase in retention time and increased water column nutrient concentrations and temperature of shallow water layers, which may lead to severe cyanobacterial blooms. This effect can potentially be counteracted by the positive response of submerged macrophytes, which compete for nutrients with cyanobacteria, with a higher chance of cyanobacterial blooms under eutrophic conditions. The balance between dominance by submerged macrophytes or cyanobacteria is temperature sensitive with stronger positive effects of drawdown as inhibition of cyanobacterial blooms expected in colder climates. Complete drying out reduces the amount of cyanobacteria in the water column after refilling, with lower water nutrient concentrations, lower fish biomass, lower abundance of cyanobacteria, higher transparency, and higher cover of submerged plants compared to lakes and reservoirs that did not dry out. Water-level rise as response to flooding has contrasting effects on the abundance of cyanobacteria depending on water quality. We conclude that water-level fluctuation management has potential to mitigate cyanobacterial blooms. However, the success will depend strongly on ecosystem properties, including morphometry, sediment type, water retention time, quality of inlet water, presence of submerged vegetation or propagules, abundance of fish, and climate.

Nutrient flux, uptake, and autotrophic limitation in streams and rivers

Assessments of biotic nutrient limitation in aquatic ecosystems typically rely on concentrations and ratios of potentially limiting nutrients. While successful in lakes, this approach has been less effective in streams, which often are dominated by benthic autotrophs (e.g., algae and vascular plants). We compared water-column nutrient concentration (C) to 3 alternative metrics and assessed their ability to predict autotrophic nutrient limitation in streams. One metric, nutrient flux (F) is the product of nutrient concentration and stream discharge. The other 2 metrics, autotrophic uptake length (Sw,a) and autotrophic uptake velocity (Vf,a), are derived from nutrient-spiraling theory and incorporate nutrient uptake. To evaluate the ability of each metric to predict nutrient limitation, we analyzed nutrient diffusing substrata (NDS) data from the Lotic Intersite Nitrogen eXperiment (LINX) (phases I and II; n = 80 streams across North America). We calculated NDS response ratios (RRs) to quantify the strength of autotrophic nutrient limitation in each stream, and we regressed RRs against each of the 4 metrics for N and P. For the LINX I analysis, Sw,a was the best predictor of autotrophic N limitation. F and Vf,a also performed well, and C was a poor predictor. In contrast, all 4 metrics were poor predictors of N limitation for the LINX II analysis when evaluated for individual streams, by land use, and within regions. None of the metrics were able to predict P limitation for either LINX study. Accuracy could be enhanced by developing new methods to quantify autotrophic nutrient uptake and limitation in stream reaches, but consistency with nutrient-spiraling theory and improved predictability of autotrophic N limitation make Sw,a a potentially useful metric for evaluating N limitation in streams.

Aquatic Macrophytes Alter Metabolism and Nutrient Cycling in Lowland Streams

Macrophytes influence the physical, chemical, and biological characteristics of lowland streams, so may be critically important in stream management. We investigated the role of macrophytes in regulating metabolism and nutrient cycling in three lowland, agricultural streams. We measured stream metabolism over the growing season and following experimental macrophyte removal, and used short-term nutrient additions of phosphate (P) and ammonium to assess macrophyte influences on nutrient uptake. Primary production was closely correlated with macrophyte cover across all streams and dates, and decreased greatly with macrophyte removal, whereas ecosystem respiration was not correlated with macrophyte cover and was not altered by macrophyte removal. Phosphate uptake velocity was negatively related to primary production, suggesting that macrophyte activity actually slowed P uptake. Ammonium uptake was not correlated with macrophyte cover or metabolism metrics. Stream nitrate concentrations typically exceeded concentrations of incoming groundwater, suggesting little net nitrate retention in these macrophyte-dominated streams. Phosphorous demand by macrophytes was 10-fold lower than observed uptake rates, indicating that macrophyte P demand was much lower than that of other stream biota. Nitrogen demand by macrophytes was nearly equal to ammonium uptake and was not sufficient to affect the high nitrate flux. These results indicate that macrophytes drive ecosystem metabolism but have limited influence on water column nutrient concentrations because macrophyte demand is much lower than the supply available from the water column. Thus macrophytes in our streams had a large impact on stream trophic state, but offered little potential to influence nutrient removal via management.

Effects of temperature, pH and nutrient concentration on branched GDGT distributions in East African lakes: Implications for paleoenvironmental reconstruction

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids found in soils and sediments and their relative abundance correlates with temperature and pH, enabling them to be used as proxies in reconstructing past climatic and environmental conditions. However, the potential for other environmental variables, such as nutrient concentration, to affect brGDGT distributions remains largely unexplored. We have examined the influence of a suite of environmental factors, including temperature, lake water and sediment chemistry, and lake morphometry on brGDGT concentration and distributions in the surface sediments of 111 lakes in East Africa. We found that temperature was the major control on the distributions, while the influence of pH was relatively minor. Water depth also had a minor but statistically significant influence, perhaps due to the relationship between lake depth and deep water anoxia. Water column nutrient concentration did not have a significant effect on the distributions or concentration. We further explored the potential for these variables to affect brGDGT temperature reconstruction by examining the correlation between them and the residuals of our brGDGT temperature calibration. We found that, while the distribution of some cyclized brGDGTs may be influenced by pH and other environmental variables, they are necessary in brGDGT calibration equations in order to accurately reconstruct temperature, likely due to covariation between temperature and other environmental variables. While surface water pH correlated with the relative abundance of certain brGDGTs, caution should be exhibited when using brGDGTs as a pH proxy because of systematic calibration errors.

Mussel farming impacts on trophic status and benthic community structure in Maliakos Gulf (Eastern Mediterranean)

The impact of shellfish farming on the water column nutrient concentration and the benthic community structure was investigated seasonally at a mussel farm (Mytilus galloprovincialis L.) of Maliakos Gulf (Eastern Mediterranean). In most cases concentration of nutrients and chl a at the farm site was lower than those observed at the control sites. Statistical analysis in concentration of all nutrients and chl a indicated significant interactions between sites and seasons, which in all cases were temporal. Furthermore, no significant interactions were found between sites and seasons for all sediment and benthic community parameters, except for Shannon–Wiener diversity index and species richness. The significant interactions detected in the biotic data were temporal. Analysis of similarity and similarity percentages analysis revealed a certain fluctuation of benthic macrofaunal community during the overall sampling year mainly due to temporal rather than spatial effects. Cluster analysis and multidimensional scaling ordination plot indicated the clearly separation of the farm site and the control sites in most of the seasons. K-dominance curves showed small difference in elevation and overlap indicating minimal difference in biological stress between the farm site and the control sites. All the above indicates that probably the farming of mussels was beneficial for the trophic status of Maliakos Gulf and that there was a minimal environmental stress caused by the shellfish farming for the surrounding benthic environment.

Seasonal variability in baseline δ15N and usage as a nutrient indicator in Lake Poyang, China

Seasonal variation in nitrogen isotope ratios (δ15N) of four baseline candidates belonging to the planktonic and benthic food webs of Lake Poyang, China, was investigated. These baseline candidates were particulate organic matter (POM), sediment organic matter (SOM), a filterer mussel (Corbicula flumine), and a grazer snail (Bellamya aeruginosa). δ15N values of POM and SOM varied significantly among the sampling seasons, while no significant seasonal variation was observed among the δ15N values of the mussels and snails sampled in different seasons. POM δ15N was correlated with lake water PO4-P, while SOM δ15N was correlated with NO3-N. Mussel δ15N and lake water TN, NO3-N, and PO4-P were significantly correlated as were snail δ15N and TN, TP, and NO3-N. SOM and snails were better bioindicators of the water column nutrient concentration of Lake Poyang. The significant correlations between mussel δ15N and POM, as well as between snail δ15N and SOM, indicated the potential use of these baselines in trophic evaluations in planktonic and benthic food webs in this water-level fluctuating lake.