Indicators Of Nutrient Enrichment Research Articles

First ecological analysis of lacustrine testate amoebae in Guatemala: A case study from the highland Lake Chichoj

Freshwater quality represents a central issue for human populations and the conservation of aquatic communities. In this sense, freshwater reservoirs, such as lakes, require proper management and monitoring plans to avoid their deterioration and pollution. Bioindicators, such as testate amoebae, are an excellent tool increasingly utilized for limnology and paleolimnology to assess the trophic status of lacustrine environments. However, despite their potential as bioindicators, the ecological research status of testate amoebae in Central American lakes remains poor. We conducted our research at highland Lake Chichoj, Alta Verapaz, Guatemala, which has become increasingly eutrophic since the 1980s. This study contributes to fill the knowledge gap about neotropical testate amoebae, parallel to testing their utility as bioindicators of lacustrine conditions. From a collection of 12 surface sediment samples (associated with different land uses), we found 19 testate amoebae taxa, and for the first time in Guatemala, we recorded Arcella megastoma, Arcella gibbosa, Cucurbitella tricuspis, Difflugia protaeiformis strain “acuminata”, Difflugia urceolata strain “elongata”, Lesquereusia spiralis, Lesquereusia modesta, and Mediolus corona. Our cluster analyses revealed three testate amoebae assemblages in connection to trophic conditions: 1) Stressed Conditions (SC), 2) Lowest Contamination Conditions (LC), and 3) Deep Transitional Conditions Assemblage (DT). After performing a transformation-based redundancy analysis (tb-RDA), we found total organic carbon as the only significant environmental parameter associated with testate amoebae assemblages (p<0.004). Our indicator species analysis (IndVal) confirms the eutrophic regime of Lake Chichoj in connection to the presence of Cucurbitella tricuspis and Centropyxis aculeata strain “aculeata” as indicators of nutrient enrichment and stressful conditions. The testate amoebae assemblages identified in Lake Chichoj represent a critical baseline for future studies of Guatemalan lakes, strengthening our understanding of the causal factors behind water quality in neotropical regions.

Assessing multiple stressor effects to inform climate change management responses in three European catchments

ABSTRACT Interactions between stressors in freshwater ecosystems, including those associated with climate change and nutrient enrichment, are currently difficult to detect and manage. Our understanding of the forms and frequency of occurrence of such interactions is limited; assessments using field data have been constrained as a result of varying data forms and quality. To address this issue, we demonstrate a statistical approach capable of assessing multiple stressor interactions using contrasting data forms in 3 European catchments (Loch Leven Catchment, UK: assessment of phytoplankton response in a single lake with time series data; Pinios Catchment, Greece: macroinvertebrate response across multiple rivers using spatial data; and Lepsämänjoki Catchment, Finland: phytoplankton response across multiple rivers using spatiotemporal data). Statistical models were developed to predict the relative and interactive effects of climate change and nutrient enrichment sensitive indicators (stressors) on indicators of ecological quality (ecological responses) within the framework of linear mixed effects models. In all catchments, indicators of nutrient enrichment were identified as the primary stressor, with climate change-sensitive indicators causing secondary effects (Loch Leven: additive, total phosphorus [TP] × precipitation; Pinios: additive, nitrate × dissolved oxygen; Lepsämänjoki: synergistic, TP × summer water temperature), the intensity of which varied between catchments and along the nutrient stressor gradient. Simple stressor change scenarios were constructed for each catchment and used in combination with mechanistic models to explore potential management responses. This approach can be used to explore the need for multiple stressor management in freshwaters, helping practitioners navigate a complex world of environmental change.

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

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

Insights to bioprocess and treatment competence of urban wetlands

Wetlands play a major role in the recharge of groundwater resources, maintenance of water quality (remediation), moderate microclimate while supporting local livelihood through provision of fish, fodder, etc. The present study aims to investigate algal-bacterial consortium as a function of residence time with the water quality dynamics in two major wetlands in Bangalore city, India. Over thirty-two genera of algae were recorded with more than 40 species in the lakes and two dominant bacterial assemblages. The higher Ammonium-N content favoured the growth of these members. Significant correlation was observed between the nutrient concentrations and the community structure at the inflows and the outflows. The algal community showed negative correlation to filterable COD and high nutrients levels while bacterial abundance was observed under high loadings. The green algae Chlorophyceae (Chlorella blooms), which are indicators of nutrient enrichment were observed predominantly, that needs an immediate attention. Higher overall treatment efficiency was observed in terms of CNP removal during the Pre-monsoon season attributed to absence of macrophytes cover and rapid growth of algal assemblage's due to higher temperature regimes with adequate solar insolation.

Refining the palaeoecology of lacustrine testate amoebae: insights from a plant macrofossil record from a eutrophic Scottish lake

Arcellinida (lobose testate amoebae) preserve well in lake sediments and have shown great potential as palaeolimnological indicators, but a limited understanding of their ecology prevents more in-depth interpretations of both contemporary and fossil assemblages. Loch Leven, Scotland, has a well-documented history of nutrient-enrichment and associated lake biological change, creating a temporal gradient along which testate amoeba ecology can be investigated. Plant macrofossil data from a dated sediment core (LEVE14) indicate that a transition from oligo-mesotrophic to eutrophic conditions occurred over at least the last 200 years, enabling direct comparisons to be drawn with fossil testate amoeba assemblages from the same core. Cluster analysis and principal components analysis show that testate amoebae and macrophytes responded synchronously to an early phase of nutrient-enrichment after ca. AD 1200, and to anthropogenic water-level lowering and further enrichment after ca. AD 1830. The results of indicator species analysis (IndVal) support the suggestion inferred previously that Cucurbitella tricuspis is an indicator of nutrient-enrichment, while the association of some less abundant taxa, specifically Difflugia amphora and Mediolus corona, with eutrophic environments is also corroborated. Overall, the testate amoeba assemblage response appears to reflect a complex interaction of factors which were ultimately driven by eutrophication. These include variations in microhabitat associated with changing plant structure and sedimentary conditions influenced by macrophyte community succession. In particular, a shift from an oligo-mesotrophic macrophyte assemblage composed of isoetid plants to one dominated by Potamogeton spp. and dense stands of Characeae is suggested to influence the relative proportions of sediment dwelling and plant-associated testate amoebae. The results highlight the utility of testate amoebae as benthic indicators of lake nutrient-enrichment and demonstrate the value of using multi-proxy palaeolimnological data to refine the ecology of lesser-studied palaeoecological groups.

Microbial genetic and enzymatic responses to an anthropogenic phosphorus gradient within a subtropical peatland

Many of the world's peatlands are subject to agricultural runoff, which may impact fundamental biogeochemical processes by altering the concentrations of limiting nutrients. In this study, the distribution of specific microbial genes associated with phosphorus (P) and nitrogen (N) metabolism was compared with potential enzyme activities related to P and N acquisition at five stations along a nutrient gradient in soils of Water Conservation Area 2A (WCA-2A) of the Florida Everglades, USA. Quantitative PCR was used to compare the relative concentrations of genes encoding alkaline phosphatases (phoX and phoD) with those encoding dinitrogenase reductase (nifH). Combined phosphatase and phosphodiesterase (EP) activities were compared with leucine aminopeptidase activities (EN), yielding a measure of potential microbial investment in P acquisition relative to N acquisition. The significant inverse relationship observed between bicarbonate extractable organic P concentrations and ratios of gene copy numbers of phoX:nifH (p=0.049, R2=0.77), and phoD:nifH (p=0.043, R2=0.79), combined with the significant inverse relationship between total P and EP/EN (p=0.021, R2=0.87), suggest that there is a greater community selection towards P acquisition relative to N acquisition as bicarbonate extractable organic P and total P decrease, suggesting a shift from P limitation to N limitation along the transect. The total number of unique genes detected by the functional microarray GeoChip 3.0 was greatest in an intermediate site, suggesting that the alleviation of nutrient limitation yielded increased functional diversity. The general agreement between genetic and enzymatic data suggests that assessing microbial nutrient demands with molecular techniques is feasible, although future work is needed to apply genetic information as indicators of nutrient enrichment.

BIOECONOMIC MODEL OF EASTERN BALTIC COD UNDER THE INFLUENCE OF NUTRIENT ENRICHMENT

Abstract The objective of this paper is to study the economic management of Eastern Baltic cod (Gadus morhua) under the influence of nutrient enrichment. Average nitrogen concentration in the spawning areas during the spawning season of cod stock is chosen to be an indicator of nutrient enrichment. The optimal cod stock is defined using a dynamic bioeconomic model for the cod fisheries. The results show that the current stock level is about half of the estimated optimal stock level and that the current total allowable catch (TAC) is about one‐fourth of the optimal equilibrium yield. The results also indicate that the benefit from a reduction in nitrogen very much depends on the harvest policies. If the TAC is set equal to the optimal equilibrium yield, the benefit of a nitrogen reduction from the 2009 level to the optimal nitrogen level would be about 604 million DKK over a 10‐year time horizon, given a discount rate of 4% per year. However, if a recovery management plan is chosen, the benefit would only be about 49 million DKK over a 10‐year time horizon.

Identification of macro-invertebrate taxa as indicators of nutrient enrichment in rivers

Eutrophication of fresh waters, especially from diffuse sources, is often a priority environmental issue for industrialised countries. Understanding the relationships between nutrient pressures and their impacts on ecology is essential for predicting the likely benefits of a programme of remedial measures to return nutrient concentrations to former levels. The aim of this study was to use mutual information to analyse the strength of association between macroinvertebrate families and nutrient levels (Total Oxidised Nitrogen and Total Reactive Phosphorus) in data covering rivers in England and Wales. Prior to the analysis the dataset was screened to minimise the confounding effects of organic pollution and split according to site type and season. Significance thresholds for the values of mutual information were calculated and the most significant indicator taxa were identified for each site type, season and nutrient pressure. It was found that in upland rivers the most significant indicator taxa were generally positive indicators, that is, their presence is indicative of high levels of at least one nutrient. In addition the number of significant indicators was greatest in upland rivers and least in lowland rivers.

Reeds as indicators of nutrient enrichment in a small temporarily open/ closed South African estuary

This study investigated the use of δ15N levels in Phragmites australis reeds as an indicator of nutrient input into the small temporarily open/closed East Kleinemonde Estuary, Eastern Cape, South Africa. Nutrient (NH4 and SRP) concentrations decreased from the bank towards the main estuary channel, suggesting that nutrients introduced into the estuary in groundwater and surface runoff were taken up by the fringe of reeds. The roots, rhizomes, stems and leaves of Phragmites at the site with the greatest Phragmites biomass and density had significantly higher δ15N (∼20‰) than the leaves at a site with no groundwater input, consistent with nutrient enrichment by septic tank wastewater and stormwater runoff. This study suggests that the δ15N level in reeds can be used to detect enrichment in small estuaries threatened by changing land use and deteriorating water quality.

Tissue nutrient concentrations in freshwater aquatic macrophytes: high inter‐taxon differences and low phenotypic response to nutrient supply

Summary1. The elemental composition and stoichiometry of aquatic plants has often been suggested to reflect the nutrient enrichment of aquatic habitats. However, the relationship is often weak. Moreover, uncertainties remain in the relevance of laboratory derived critical plant tissue nutrient concentrations to maximum yield or growth rates in the field.2. Aquatic vascular plants and bryophytes, overlying water and sediment samples were collected to test whether freshwater aquatic macrophytes: (i) show tissue nutrient deficiencies when growing in oligotrophic freshwater habitats, and (ii) have strict homeostatic stoichiometry.3. Plant nutrient concentrations were significantly related to total inorganic nitrogen (or nitrate), total dissolved phosphorus and sediment total phosphorus. However, these relationships were weak. Virtually all the variance in plant tissue nutrient concentrations, however, could be explained by species (taxon) identity.4. Critical tissue nutrient concentrations for 95% maximum yield or 95% maximum growth rate in aquatic angiosperms, determined from laboratory bioassays, suggested that nutrients should not limit yield in wild aquatic macrophytes. However, there were a substantial number of samples where potential growth rate limitation was possible, particularly due to phosphorus.5. Strict C : N : P stoichiometric ratios were found for both vascular plants and bryophytes, suggesting little scope for plants as indicators of nutrient enrichment, but provide robust stoichiometric data for studies on ecosystem metabolism and nutrient cycling.

Polychaete assemblages as indicators of habitat recovery in a temperate estuary under eutrophication

From 1980 to 2000, the Mondego estuary (Portugal) suffered dramatic changes due to eutrophication, leading to a decline in seagrass beds and changes in community structure, namely a decline in species richness and replacement of herbivores by detritivores. Because the conservation status of the estuary was being compromised, a restoration project was implemented in 1998 in order to restore the original natural seagrass habitat. This project included several restoration/management measures, including the protection of the remaining seagrass patches and improvement of the hydraulic conditions through modifying sluice opening regimes, thereby reducing the nutrient loading in the southern arm of the estuary. In addition, several stakeholder forums were convened to inform local people of the ecological and economic importance of the area. In this paper we evaluate the success of this management plan as judged by changes in the polychaete assemblages. In the year following implementation of the restoration measures, the total biomass of polychaetes increased markedly, a pattern also seen in the overall macrobenthic community. Biomass recovery was faster and more pronounced compared to density recovery, probably because many larger species were able to attain adult body size (e.g. Hediste diversicolor). The polychaetes Alkmaria romijni and Capitella capitata, considered as indicators of nutrient enrichment and pollution of estuarine systems, gradually declined over time. A general increase in species diversity was observed after the application of the restoration/management measures. These changes indicate that the restoration project has been successful and that polychaetes can be considered representative indicators of the wider macrobenthic community, with considerable savings in time and effort for monitoring programmes.

Non-pollen microfossils in Everglades sediments

Modifications of the hydrology of the Florida Everglades began in the 1880s. Impacts due to changes in the hydrological regime, as well as nutrient enrichment are clear in the northern Everglades. Although these changes were more subtle in the southern Everglades, altered hydroperiods and plant communities are targeted for restoration. Identification of restoration goals requires knowledge of past conditions, but detailed vegetation surveys prior to the pre-drainage era are lacking. Palynological studies can help fill this void. Interpretations based upon pollen and plant spores alone are limited by low pollen production of the dominant marsh species and low plant diversity in the ecosystem. In some regions the occurrence of other microfossils present in palynological preparations has been used to help interpret hydrological regimes and nutrient status and here we explore that potential for paleoecological records from the Everglades. In this study we report the distribution of various microfossils found within pollen zones identified in sediments cored from three tree islands in Shark River Slough, Everglades National Park. Using published modern pollen analogues we classified paleovegetation zones as slough, sawgrass marsh, tree island or tree island edge and documented the abundances of microfossils of algae, fungi, cyanobacteria and testate amoebae. Tree island zones, indicative of relatively short hydroperiods, were characterized by highest abundances of fungal material, testate amoebae and the base of the deposits, by the cyanobacterium, Gloeotrichia. Much of the fungal material appears to be associated with plant roots, and the mycorrhizal Glomus is locally abundant. The taxonomic affinity of the testate amoebae has not been determined, but some were arcellaceous forms that included charcoal particles in their tests. Gloeotrichia also is found in marsh and slough deposits. Its occurrence in the latter probably signals minimal water depth. Botryococcus and Zynemataceae zygospores are found in all deposits, but are less abundant in the tree island deposits. Both groups are probably part of the periphyton flora, but Botryococcus has not been recorded in phycological studies of the Everglades. Observation of these algae in the seasonally dry Everglades is consistent with other studies where they have been reported to represent shallow, slow moving waters, but inconsistent with the assumptions of meso- or eutrophic conditions commonly made in other studies. Our observation of the alga Botryoccocus and the meroplanktonic cyanobacterium Gloeotrichia in the historically oligotrophic Everglades challenges their status as indicators of nutrient enrichment.

Evaluation and correction of subresolved particles by the optical plankton counter in three Australian estuaries with pristine to highly modified catchments

High concentrations of subresolved particles smaller than the 250 μm equivalent spherical diameter (ESD) detection limit of the optical plankton counter (OPC) have hampered its use in turbid estuarine waters. Coincidence of these subresolved particles produced erroneous counts of up to 58 L−1 for 100 μm mesh filtered water samples from three subtropical east Australian estuaries using the laboratory OPC‐1L. The influence of these erroneous counts on in situ OPC‐2T measurements was assessed by comparison with measurements of simultaneously collected net zooplankton using the laboratory OPC‐1L. The total zooplankton abundance from the in situ OPC‐2T measurements could be corrected for erroneous counts of subresolved particles using OPC‐1L measurements of 100 μm mesh filtered water sampled from the same site but with large error. No such corrections were possible for OPC‐2T measurements of total zooplankton biomass or normalized biomass size spectra (NBSS). No meaningful or significant correlations were found between the abundance or biomass of subresolved particles and in situ light attenuance, probably due to tannin‐rich waters with low subresolved particle concentrations. NBSS of simultaneously collected net zooplankton from OPC‐1L measurements indicated higher biomass in the disturbed Manning and Wallamba rivers, whose catchments support intensive livestock agriculture and some residential development, compared to the forested Wallingat River. NBSS may therefore be a useful indicator of nutrient enrichment in estuaries. The slope of NBSS may respond to both production of small particles and the predation and loss of large particles.

Benthic diatoms in USA rivers: distributions along spatial and environmental gradients

AimTo study large‐scale patterns of benthic diatom assemblages in rivers, to assess the relative importance of environmental and geographical factors affecting their composition, and to evaluate the implications of these patterns for the use of diatoms as indicators of water quality, particularly nutrient enrichment.LocationThe United States Geological Survey (USGS) National Water Quality Assessment (NAWQA) Programme data set covers the conterminous United States.MethodsWe employed gradient analysis to understand the floristic structure of the data set and to discover major ecological gradients underlying variation in species composition at different spatial scales (entire US, Omernik Level 1 and Level 2 ecoregions). We used variance partitioning to separate the effect of environmental and spatial characteristics.ResultsAt the national scale, three major ecological gradients were evident. The first was a complex `downstream' gradient from fast‐flowing, mostly oligotrophic, highland rivers to predominantly eutrophic rivers of high‐ and low‐elevation plains. The second was a gradient integrating water mineral content and pH, which separated the soft and often more acidic waters of the humid eastern part of US from the alkaline waters of arid western regions. The third obvious gradient was related to latitudinal and altitudinal variation of temperature. Up to one‐third of the total explainable variation in species data was attributed solely to geographical factors not correlated with measured environmental characteristics. We present several examples of species with complex patterns of spatial distribution.Main conclusionsAlthough environment consistently plays the most important role in structuring diatom assemblages in rivers, spatial factors also explain some variation in diatom distribution, especially at the continental scale. Most of the species that are confined to limited geographical areas are not yet described and await future taxonomic work. We show that selection of species which could serve as indicators of nutrient enrichment in US rivers is not a straightforward procedure. The existence of complex environmental gradients, and still poorly understood spatial patterns of species distribution, precludes attempts to develop uniform diatom‐based metrics that would be applicable everywhere in the US. We advocate the development and calibration of metrics based on data sets collected from more limited geographical areas, and that include sites having relatively narrow ranges of environmental characteristics other than that which the metrics are designed to indicate.

Using Winter Flounder Growth Rates to Assess Habitat Quality across an Anthropogenic Gradient in Narragansett Bay, Rhode Island

We used growth rates of juvenile winter flounderPseudopleuronectes americanus to assess anthropogenic influence on habitat quality at three sites in Narragansett Bay, Rhode Island. The upper bay site, Gaspee Point, had the highest population density and concentration of total nitrogen; human inputs decreased down bay. Growth rates of individually marked fish were measured in three 15-d experiments from June 8 to July 6, 1998 in 1-m2 cages placed at upper, middle, and lower bay sites. Water temperature, salinity, dissolved oxygen (DO), and benthic food were also measured. Stable isotopes of nitrogen and carbon were measured in experimental fish as possible indicators of nutrient enrichment and to identify organic carbon sources. Growth rates were 0.22–0.60 mm d−1, with the highest average at the mid-bay site. Growth was initially fastest at Gaspee Point, but dropped off as DO concentrations fell. Step-wise multiple regression indicated that location (upper, middle, or lower bay) explained most of the variability in fish growth (40%). Coefficients of other significant variables indicated that fish grew faster at lower salinities, smaller sizes, and with decreased time that DO was below 2.3 mg l−1. Benthic prey varied among sites and there was significantly less food and fewer species at Gaspee Point.Polydora cornuta was a favored food at all sites and was found in over half the stomachs. Values of δ15N in fish and sediments did not reflect differences in total nitrogen concentrations recorded near the sites. We suggest that anthropogenic influences, such as nutrients and sewage, affected habitat quality by reducing DO, which lowered fish growth rates.

Effects of urbanization on streams of the Melbourne region, Victoria, Australia. II. Benthic diatom communities

1. Epilithic and epiphytic diatom community composition were assessed in small streams of the Melbourne region to test the effects of (a) urban density (sub‐catchment imperviousness 0–51%) and (b) stormwater drainage intensity (comparing the intensively drained metropolitan area with urban areas of the hinterland, which had open drains and some localized stormwater drainage).2. Communities separated into three groups: eastern hinterland, western hinterland and a metropolitan group. Separation of eastern and western hinterland groups, and of eastern and western sites within the metropolitan group were best explained by patterns of electrical conductivity, basalt geology and annual rainfall. Separation of metropolitan and hinterland groups, and patterns within the hinterland groups were best explained by nutrient gradients (phosphorus, ammonia and total nitrogen).3. Nutrient concentrations were not only apparently influenced by urban density but also by effluents from small sewage treatment plants and agricultural activities at a few sites.4. Species richness did not vary consistently between the metropolitan and hinterland groups but within the western hinterland, sites with low nutrient concentrations tended to be more species‐rich than mildly enriched sites.5. Composition of both diatom and macroinvertebrate communities (assessed in a concurrent study) were sensitive indicators of urban‐derived impacts. However, diatoms were better indicators of nutrient enrichment, while macroinvertebrates were better integrative indicators of catchment disturbance.

Assessing Impacts of Past Human Activity on the Water Quality of Upper Saranac Lake, New York

ABSTRACT Paleolimnological techniques were used to establish baseline environmental conditions as guidelines for lake management and to evaluate water quality changes over the last 300 years in the north basin of Upper Saranac Lake (Adirondack Park, New York). Analysis of diatoms, bulk sedimentary characteristics and fossil pigments in a 30-cm core showed that the lake was once oligo- to mesotrophic but has become increasingly productive since the turn of the century, especially since the early 1950's. Relative abundances of the diatoms Fragilaria crotonensis, Tabellaria flocculosa, and Asterionella formosa – common indicators of nutrient enrichment – increased in the most recent sediments, as did loss-on-ignition, cyanobacterial pigments, and inferred total phosphorus, pH, and chloride. Comparison of core profiles with historical data suggests that the productivity increased because of human activities, in eluding shoreline development and the discharge of fish hatchery effluent into a major tributary of the north basin. Short-term variations in diatom-inferred phosphorus since 1950 may be partly attributable to weather conditions.

Role of Benthic Plants in Fertilized Estuary

A study to elucidate the productivity contribution of benthic macrophytes in a fertile estuary was undertaken. Methods for utilizing C\U14\N uptake as a productivity monitor of benthic plants are described. Both phytoplankton and macrophyte productivity determinations were made at various distances from a small sewage outfall, and the effect of nutrients on the phytoplankton and seaweeds was observed. It was also observed that the benthic flora was a better indicator of nutrient enrichment, because there was a uniform gradient of productivity stimulation away from the outfall. The phytoplankton showed erratic values at various distances from the outfall due to mixing in and out of the fertilized zone. It is proposed that a productivity model such as the Baule-Mitscherlich relationship can be used to predict the photosynthetic response of algae due to a combination of nutrients. This model will allow for the concept of multiple limiting nutrients in nature.