Dynamics Of Aquatic Communities Research Articles

Freshwater connectivity transforms spatially integrated signals of biodiversity.

Aquatic ecosystems offer a continuum of water flow from headwater streams to inland lakes and coastal marine systems. This spatial connectivity influences the structure, function and dynamics of aquatic communities, which are among the most threatened and degraded on the Earth. Here, we determine the spatial resolution of environmental DNA (eDNA) in dendritic freshwater networks, which we use as a model for connected metacommunities. Our intensive sampling campaign comprised over 420 eDNA samples across 21 connected lakes, allowing us to analyse detections at a variety of scales, from different habitats within a lake to entire lake networks. We found strong signals of within-lake variation in eDNA distribution reflective of typical habitat use by both fish and zooplankton. Most importantly, we also found that connecting channels between lakes resulted in an accumulation of downstream eDNA detections in lakes with a higher number of inflows, and as networks increased in length. Environmental DNA achieves biodiversity surveys in these habitats in a high-throughput, spatially integrated way. These findings have profound implications for the interpretation of eDNA detections in aquatic ecosystems in global-scale biodiversity monitoring observations.

Effect of environmental integrity on the functional composition of the Odonata (Insecta) community in streams in the Eastern Amazon

Anthropic activities affect the dynamics of aquatic communities and can influence the reproductive behavior of many species. In addition, functional diversity is expected to be influenced by the environment. In this context, we evaluated how the biological functional characteristics of the Odonata adult community respond to impacts caused by human action on streams in the Eastern Amazon, using bionomic characteristics as response variables. Concomitantly, we analyzed which characteristics are responsible for the presence of species in the environment. We sampled adults of Odonata in 98 preserved and altered streams in the Eastern Amazon. We used as functional characteristics: oviposition, thermoregulation and body size, and as morphological characteristics: width of the thorax, width of the wing at the base, length of the abdomen and length of the thorax. We recorded 80 species, distributed in 16 functional groups and three categories: present in all environments, present only in preserved environments, and present only in altered environments. There was variation in the functional characteristics studied between the environments (PerMANOVA; F = 15,655; P < 0.01), with a significant difference in the composition of attributes between the environments studied. Although PCoA did not find a strong relationship between the functional attributes and the level of integrity, the individuals found in altered areas are heliothermic, exophytic oviposition, with a wider wing width at the base and larger size. Individuals with smaller body size and endophytic and epiphytic oviposition, and thermal and endothermic conformators are found in preserved areas. Our study provides evidence that functional attributes are determining factors for the occurrence of species in the environment. The high quality of environment has a significant effect on the composition of functional groups. Exophytic and heliothermic species are favored by altered environments, while in preserved environments, the species that are best adapted are those that present epiphytic and endophytic oviposition and ectothermic thermoregulation (thermal conformers). As for morphology, altered environments favor medium to large individuals, with greater thorax length and abdomen size, preserved environments may favor the smaller and/or specialized species.

Integrating the flow regime and water quality effects into a niche-based metacommunity dynamics model for river ecosystems

Aquatic community dynamics are closely dominated by flow regime and water quality conditions, which are increasingly threatened by dam regulation, water diversion, and nutrition pollution. However, further understanding of the ecological impacts of flow regime and water quality conditions on aquatic multi-population dynamics has rarely been integrated into existing ecological models. To address this issue, a new niche-based metacommunity dynamics model (MDM) is proposed. The MDM aims to simulate the coevolution processes of multiple populations under changing abiotic environments, pioneeringly applied to the mid-lower Han River, China. The quantile regression method was used for the first time to derive ecological niches and competition coefficients of the MDM, which are demonstrated to be reasonable by comparing them with the empirical evidence. Simulation results show that the Nash efficiency coefficients for fish, zooplankton, zoobenthos, and macrophytes are more than 0.64, while the Pearson correlation coefficients for them are no less than 0.71. Overall, the MDM performs effectively in simulating metacommunity dynamics. For all river stations, the average contributions of biological interaction, flow regime effects, and water quality effects to multi-population dynamics are 64%, 21%, and 15%, respectively, suggesting that the population dynamics are dominated by biological interaction. For upstream stations, the fish population is 8%–22% more responsive to flow regime alteration than other populations, while other populations are 9%–26% more responsive to changes in water quality conditions than fish. For downstream stations, flow regime effects on each population account for less than 1% due to more stable hydrological conditions. The innovative contribution of this study lies in proposing a multi-population model to quantify the effects of flow regime and water quality on aquatic community dynamics by incorporating multiple indicators of water quantity, water quality, and biomass. This work has potential for the ecological restoration of rivers at the ecosystem level. This study also highlights the importance of considering threshold and tipping point issues when analyzing the “water quantity-water quality-aquatic ecology” nexus in future works.

Freshwater gastropods as an important group for studying the impact of inter- and intra-specific chemical communication on aquatic community dynamics

Freshwater gastropods as an important group for studying the impact of inter- and intra-specific chemical communication on aquatic community dynamics

Reservoir ecological operation by quantifying outflow disturbance to aquatic community dynamics

Reservoir operation causes spatiotemporal variations in outflow, which influence the dynamics of downstream aquatic communities. However, empirical evidence of community responses to hydrological alteration remains limited for dam-regulated rivers. This study focused on quantifying the streamflow disturbance to multi-population dynamics in downstream of the China’s Danjiangkou Reservoir. First, the stochastic population dynamics model (PDM) was used to simulate aquatic community dynamics. Then, the flow–ecology relationship was established to identify community response to reservoir outflow. Third, two novel ecological indicators, stable time (ST) and coefficient of variation at stable time (CVST), were proposed to evaluate the resilience and resistance of multi-population systems, respectively. Finally, the reservoir operating rule curves were optimized by considering tradeoffs between socioeconomic and ecological objectives. The coevolution processes of multi-population systems (fish, phytoplankton, zooplankton, zoobenthos, and macrophytes) were simulated by stochastic PDMs. The population densities of stable states showed continuous downward trends with increasing degree of hydrological alteration for multi-population systems, and aquatic community systems could be destroyed when alteration reached its acceptable maximum. The greater the degree of hydrological alteration, the longer the recovery time from an unstable to a stable state, and the weaker resistance for each population system. The resilience and resistance of downstream multi-population systems were enhanced by optimizing reservoir outflow. The optimization results illustrated that the performances of the multiple objectives of water supply, hydropower generation, and ST were improved by 2.37%, 2.40%, and 2.67%, respectively, whereas the performance of CVST was the same as the conventional operation. The flow–ecology relationship provided an approach to quantify the impacts of reservoir outflow on an aquatic community, which is helpful in guiding ecological flow strategies.

Ephemeral habitat supports high fish α‐diversity and β‐diversity during drought in a subtropical semiarid wetland

ABSTRACTGlobally, the number of rivers with intermittent flow is increasing due to climate change and water abstraction for human consumption. Currently, our understanding of how hydrology in subtropical rivers with intermittent flow affects the structure and dynamics of aquatic communities is poorly understood. Here, we investigated how fish α‐ and β‐diversity patterns in intermittent channels in the lower reaches of the Okavango Delta respond to seasonal flooding and drought. Under low‐flow conditions, ephemeral habitat had higher α‐diversity, and this was influenced by a combination of fish aggregation and apparent transient legacy effects as habitat patches became smaller and more isolated. Investigation of changes in fish assemblage structure across different hydrological periods and habitat types showed significant species turnover when water levels fell, suggesting a strong influence from species sorting. During high‐water periods, species assemblages were homogenized both at local and regional scales, suggesting a greater influence of mass effects. Our findings support hydrology as a major factor regulating diversity patterns in intermittent rivers of a major wetland in a semiarid region of subtropical Africa. We infer from these findings that maintenance of a relatively natural flow regime will be necessary for conserving aquatic ecosystem structure and function in this system.

Chronic Exposure to High Concentrations of Road Salt Decreases the Immune Response of Dragonfly Larvae

Salinization of freshwater ecosystems, due to the application of road salts, is recognized as a potential threat to aquatic communities. Much of the research on the impact of salinity has focused on performance metrics in vertebrates, including respiration and osmoregulation. Here we focus on immune function in of the dragonfly Anax junius, a top predator in fishless aquatic habitats. Impacts on this top predator have the potential to cascade through the community, and immune function is known to be both plastic and sensitive to stress. We injected larvae with monofilaments (simulating a parasite) and placed them in one of three environmentally relevant concentrations of deicing road salt: control (dechlorinated tap water with no added salt), low (1000 mgL-1), or high salt (3000 mgL-1), for either acute (24hr) chronic or (96hr) exposures. We hypothesized that elevated salinity would suppress the immune response and that longer exposure magnifies this effect. As predicted, chronic exposure to high salt concentrations resulted in a significantly reduced larval immune response, however, there was no detectable treatment effects in larvae exposed to low concentrations of road salt or to acute high concentrations. Our results demonstrate that prolonged exposure to high levels of road salt can compromise the immune response of dragonfly larvae. Our findings suggest that insects in aquatic environments that experience sustained environmental salt pollution will be more susceptible to parasites and pathogens, which in turn may affect the impact of this major predator on aquatic community dynamics.

A protocol for the analysis of aquatic biodiversity by multiple β diversities

Species assemblage can be fairly unstable in aquatic ecosystems because of strong environmental constraints. Measuring β diversity is one of the most useful tools for assessing community dynamics and differentiations among communities. Whereas many β diversity measurements have been proposed for these issues, we present here a unifying framework based on the properties of Hill number, q, to unravel the complex changes occurring in dynamic communities. Using ideal numerical examples of an aquatic community, the sensitivity of β diversity with q = 0 ∼ 2 was examined. Whereas β diversity for q = 0 has a higher sensitivity to rare species drift (rapid exchanges or migration of many rare species) than do others, β diversity for q = 2 adequately responds to the variation or changes of dominant species. A scatterplot based on these two β diversity measures is proposed for the detailed examination of aquatic community dynamics. We show that the scatterplot is able to describe the monthly dynamics of dominant and rare species in the phytoplankton assemblage of a freshwater lake in Japan.

Lentification in Alpine rivers: patterns of diatom assemblages and functional traits

Hydrological alterations in Alpine rivers have been largely increasing due to the combined effects of global climate change and local impacts. Water flow intermittency starts with a process called lentification, which leads to the transformation of the aquatic ecosystem with severe consequences on biotic communities. These consequences have a greater impact in Alpine headwater streams, which are considered important and fragile hotspots of biodiversity and a source of threatened species; the new regime creates conditions to which native biota are poorly adapted. The main aim of our work was to evaluate taxonomical, functional and structural changes of diatom communities in Alpine rivers during the lentification process that precedes a supra-seasonal drought. Contrary to what was expected, communities exposed to river shrinking showed a level of homogeneity comparable to those collected in permanent sections. We therefore hypothesized that lentification created an intermediate disturbance that favoured heterogeneity of assemblages. Conversely, ecological guilds, growth forms and eco-morphological groups varied along temporal and spatial gradients driven by the lentification process. Small, motile and medium-sized low profile species were more abundant during summer, as well as adnate and stalked ones. Lentification limited the presence of high profile guilds and mucous-forming colonies. Droughts in Alpine streams are fairly recent processes, and the dynamics of aquatic communities facing water scarcity are largely unknown. The results we obtained provide important insights towards a more refined functional classification of diatoms for evaluating hydrological alterations.

Patterns of long-term dynamics of aquatic communities and water quality parameters in the Great Lakes: Are they synchronized?

Long time series data can provide insights into dynamics of large lakes. We use the USEPA-GLNPO biological monitoring dataset (phytoplankton, benthos, zooplankton and water quality), collected from 1996 through 2016, to identify whether there is evidence of concordant linear or non-linear trends in community composition, density/biomass/biovolume and major environmental parameters. We show changes in biotic assemblages and water quality variables, particularly in Lakes Michigan and Huron. These include changes in phytoplankton biovolume and zooplankton biomass, increasing invasive Dreissena abundance and decreasing densities of other benthos. Biotic changes are accompanied by pronounced changes in water quality and nutrient ratios. Species change-points, identified using threshold indicator taxon analysis, are often less abrupt, but there are clear shifts in a large proportion of species in each assemblage. The concordance of breakpoints among assemblages or lack thereof provides valuable insight into potential drivers of ecosystem change.

Fish functional diversity responses following flood pulses in the upper Paraná River floodplain

AbstractFlood pulses are the main force driving the dynamics of aquatic communities in floodplains. The responses of communities to environmental changes following flood pulses usually demand a time lag to appear and reach the climax. We assembled a data set of 16 years of fish samplings to assess the relationship between water level and four functional diversity measures, in the upper Paraná River floodplain. Specifically, we approached four aspects of each relationship between water level and functional diversity: nature (positive or negative), sensitivity (response intensity), responsiveness (response delay) and extent (response duration). The nature of the relationship between water level and functional diversity was positive in all cases. Functional richness (FRic) responded right after flood pulses, although with shorter extent. Abundance‐dependent functional measures (evenness—FEve; divergence—FDiv; and Rao's quadratic entropy—Rao's Q) presented delayed responses, reaching peaks more than 1.5 years after flood pulses. Significant effects of floods on fish functional diversity were observed for more than 3 years, although the highest functional diversity was observed with 1.8 years, on average. More importantly, flood pulses had no longer significant effects on functional diversity after 4 years. Regarding conservation strategies in regulated systems, flood events should occur every 2 or 3 years, with adequate timing (October‐November), intensity (up to 450 cm) and duration (at least 50 uninterrupted days). Intervals longer than 3 years or inadequate timing, intensity and duration could dramatically decrease functional diversity and compromise ecosystem services.

A standardized tritrophic small-scale system (TriCosm) for the assessment of stressor-induced effects on aquatic community dynamics.

Chemical impacts on the environment are routinely assessed in single-species tests. They are employed to measure direct effects on nontarget organisms, but indirect effects on ecological interactions can only be detected in multispecies tests. Micro- and mesocosms are more complex and environmentally realistic, yet they are less frequently used for environmental risk assessment because resource demand is high, whereas repeatability and statistical power are often low. Test systems fulfilling regulatory needs (i.e., standardization, repeatability, and replication) and the assessment of impacts on species interactions and indirect effects are lacking. In the present study we describe the development of the TriCosm, a repeatable aquatic multispecies test with 3 trophic levels and increased statistical power. High repeatability of community dynamics of 3 interacting aquatic populations (algae, Ceriodaphnia, and Hydra) was found with an average coefficient of variation of 19.5% and the ability to determine small effect sizes. The TriCosm combines benefits of both single-species tests (fulfillment of regulatory requirements) and complex multispecies tests (ecological relevance) and can be used, for instance, at an intermediate tier in environmental risk assessment. Furthermore, comparatively quickly generated population and community toxicity data can be useful for the development and testing of mechanistic effect models. Environ Toxicol Chem 2018;37:1051-1060. © 2017 SETAC.

Prescribed burn creates pulsed effects on a wetland aquatic community

Fire in uplands and wetlands results in a release of nutrients and increased light in the burned area. However, fire effects on aquatic community dynamics are not well understood. We hypothesized that the addition of light and nutrients resulting from prescribed burns in wetlands increases periphyton biomass and supports increased standing stock of marsh fishes. In the oligotrophic Everglades of Florida, USA, we conducted a 2 × 2 factorial experiment using prescribed burns over standing water (increased nutrients and light), mowing with removal of above-water vegetation (no nutrient increase), and shade houses (no light) to test the prediction that fire effects would lead to more periphyton biomass and greater abundance and size of fish compared to other treatments. We observed increased periphyton percent cover and biomass per area in response to fire treatments. Fish abundance showed a short-term increase in burned plots. Fish length, mass, and condition factor did not respond consistently to treatments, though some species responded to specific treatments. Wildfires in dry marshes that may combust organic soils and vegetation may impact wetlands more than prescribed burns in flooded marshes. Our study suggests that wetland fires can affect aquatic animal and plant community structure, at least for short periods post-fire.

Effects of nutrient supplementation on host-pathogen dynamics of the amphibian chytrid fungus: a community approach.

Anthropogenic stressors may influence hosts and their pathogens directly or may alter host-pathogen dynamics indirectly through interactions with other species. For example, in aquatic ecosystems, eutrophication may be associated with increased or decreased disease risk. Conversely, pathogens can influence community structure and function and are increasingly recognised as important members of the ecological communities in which they exist.In outdoor mesocosms, we experimentally manipulated nutrients (nitrogen and phosphorus) and the presence of a fungal pathogen, Batrachochytrium dendrobatidis (Bd), and examined the effects on Bd abundance on larval amphibian hosts (Pseudacris regilla: Hylidae), amphibian traits and community dynamics. We predicted that resource supplementation would mitigate negative effects of Bd on tadpole growth and development and that indirect effects of treatments would propagate through the community.Nutrient additions caused changes in algal growth, which benefitted tadpoles through increased mass, development and survival. Bd-exposed tadpoles metamorphosed sooner than unexposed individuals, but their mass at metamorphosis was not affected by Bd exposure. We detected additive rather than interactive effects of nutrient supplementation and Bd in this experiment.Nutrient supplementation was not a significant predictor of infection load of larval amphibians. However, a structural equation model revealed that resource supplementation and exposure of amphibians to Bd altered the structure of the aquatic community. This is the first demonstration that sublethal effects of Bd on amphibians can alter aquatic community dynamics.

Contrasting copepod community dynamics related to sampling strategies in the unsaturated zone of a karst aquifer

The sampling effect may reflect artifactual trends in species richness and distribution across various spatiotemporal scales. The paper aims to examine how sampling frequency impacts the observed community dynamics in groundwater habitats of the unsaturated zone of the Ciur Izbuc cave (northwestern Romania) and addresses data interpretation biases that can arise when different sampling strategies are applied. The patterns obtained by monthly sampling over a year were compared with the patterns from the seasonal samplings over 4 years and with those from sampling conducted at an interval of 2 days over 1 week. Sample-based rarefaction curves were computed to compare estimated species richness between datasets with different sampling frequencies. Self-organizing maps were used to explore the impact of sampling frequency on depicted patterns over various temporal frames. Sampling frequency contributed to contrasting species richness, abundances and spatiotemporal dynamics of copepods in the studied groundwater habitats across scales. Monthly sampling and seasonal sampling revealed maximum number of species (seven) and led to stable species estimates after 11 sampling occasions and 12 sampling occasions, respectively, while 1 week of every-other-day sampling was not enough to capture the highest number of species. Spatial heterogeneity of species distribution increased over 4 years of seasonal sampling. Our results demonstrated the importance of sampling rate and temporal frames in understanding depicted aquatic community dynamics a must before drawing conclusions. We advocate cautious data interpretation and pattern generalization in groundwater ecology to avoid hasty assessments of community changes across spatial and temporal scales.

Climatic Influence on the Phytoplankton Communities of the Upper Reaches of the Southern Bug River (Ukraine)

ABSTRACT A total of 98 taxa of algae were observed in phytoplankton, sampled monthly, from the Khmelnitsky monitoring station in the Southern Bug River, Ukraine, between April 2010 and March 2011. Chlorophyta species are the richest taxonomic group with 46 taxa, followed by Bacillariophyta, Euglenophyta, Cyanoprokaryota, Dinophyta, Chrysophyta, Streptophyta, and Xanthophyta. Seasonal dynamics of species distribution in taxonomic divisions shows that the role of Bacillariophyta in communities was high in January-March, which were replaced by greens in March-September. Euglenoids were developed in February-December and bluegreen algae in summer communities only. Strong positive correlations between temperature and species richness was observed. Abundance and biovolume of phytoplankton were maximal in summer, caused mostly by Dolichospermum flos-aquae (Lyngb.) Wacklin, Hoffmann and Komarek and Ceratium hirundinella (O. Müll.) Bergh. The river ecosystem has two periods of trophic levels - high at summer and low at winter. Bioindication characterizes the river as low alkaline and low mineralized with a moderate organic pollution level, revealed aspects of seasonal changes and revealed the main source of organic pollution as flowing from the catchment area during ice melting and rains. Organic pollution indices fluctuate within narrow limits suggesting relative stability of the river ecosystem that is shown also by Shannon indices. The calculated indices, comparative statistics, CCA, and bio-indication analysis exhibits a low pollution level in the Khmelnitsky monitoring station that can be used as a model of aquatic community dynamics under seasonal fluctuation in the southern boreal province climate, applicable for monitoring of the Southern Bug River.

The influence of the monsoon climate on phytoplankton in the Shibpukur pool of Shiva temple in Burdwan, West Bengal, India

AbstractA total of 44 taxa were observed in monthly sampled phytoplankton of the Shibpukur pool in Burdwan, West Bengal between March, 2010 and February, 2011. The most abundant taxa belong to Charophyta, followed by cyanobacteria, diatoms and euglenoids. Bio-indication shows that the pool community preferred low alkaline, low mineralized and low organically polluted water. The total phytoplankton density showed its maximum values in May, 2010. The Shannon-Weaver diversity index and the Pielou evenness value were found to be highest during the post-monsoon season. The Simpson dominance index and the Margalef index of richness were highest in the pre-monsoon season. The total phytoplankton density showed a highly significant positive correlation with pH and salinity and significant positive correlation with air temperature, water temperature, dissolved oxygen and total suspended solids. Among the reported 44 phytoplankton taxa only 3 showed a random distributional pattern. The Bray-Curtis Cluster analysis and the comparative statistics reveal two groups of phytoplankton assemblages in respect to the monsoon seasons. The successive communities form a continuum corresponding to Colwell’s Constancy (C) category. The calculated indices, CCA, and bio-indication analysis exhibit a low pollution level in the Shibpukur pool that can be used as a model of aquatic community dynamics under seasonal fluctuation in the monsoon climate, applicable for monitoring of water bodies in the West Bengal Province

Temporal dynamics of aquatic communities and implications for pond conservation

Conservation through the protection of particular habitats is predicated on the assumption that the conservation value of those habitats is stable. We test this assumption for ponds by investigating temporal variation in macroinvertebrate and macrophyte communities over a 10-year period in northwest England. We surveyed 51 ponds in northern England in 1995/6 and again in 2006, identifying all macrophytes (167 species) and all macroinvertebrates (221 species, excluding Diptera) to species. The alpha-diversity, beta-diversity and conservation value of these ponds were compared between surveys. We find that invertebrate species richness increased from an average of 29.5 species to 39.8 species between surveys. Invertebrate gamma-diversity also increased between the two surveys from 181 species to 201 species. However, this increase in diversity was accompanied by a decrease in beta-diversity. Plant alpha-, beta- and gamma-diversity remained approximately constant between the two periods. However, increased proportions of grass species and a complete loss of charophytes suggests that the communities are undergoing succession. Conservation value was not correlated between sampling periods in either plants or invertebrates. This was confirmed by comparing ponds that had been disturbed with those that had no history of disturbance to demonstrate that levels of correlation between surveys were approximately equal in each group of ponds. This study has three important conservation implications: (i) a pond with high diversity or high conservation value may not remain that way and so it is unwise to base pond conservation measures upon protecting currently-speciose habitats; (ii) maximising pond gamma-diversity requires a combination of late and early succession ponds, especially for invertebrates; and (iii) invertebrate and plant communities in ponds may require different management strategies if succession occurs at varying rates in the two groups.

ANALYSIS OF CLIMATE CHANGE SCENARIOS BASED ON MODELLING OF THE SEASONAL DYNAMICS OF A DANUBIAN COPEPOD SPECIES

Climate change is one of the most crucial ecological problems of our age with great influence. Seasonal dynamics of aquatic communities are — among others — regulated by the climate, especially by temperature. In this case study we attempted the simulation modelling of the seasonal dynamics of a copepod species, Cyclops vicinus , which ranks among the zooplankton community, based on a quantitative database containing ten years of data from the Danube's God area. We set up a simulation model predicting the abundance of Cyclops vicinus by considering only temperature as it affects the abundance of population. The model was adapted to eight years of daily temperature data observed between 1981 and 1994 and was tested successfully with the additional data of two further years. The model was run with the data series of climate change scenarios specified for the period around 2070- 2100. On the other hand we looked for the geographically analogous areas with the God region which are mostly similar to the future climate of the God area. By means of the above-mentioned points we can get a view how the climate of the region will change by the end of the 21 st century, and the way the seasonal dynamics of a chosen planktonic crustacean species may follow this change. According to our results the area of God will be similar to the northern region of Greece. The maximum abundance of the examined species occurs a month to one and a half months earlier, moreover larger variances are expected between years in respect of the abundance. The deviations are expected in the direction of smaller or significantly larger abundance not observed earlier.

Corrigendum to: Effects of sewage discharges on microbial components in tropical coastal waters (Senegal, West Africa)

Coastal ecosystems are frequently submitted to anthropogenic pressure but little is known about how the dynamics of aquatic communities can be altered. The impact of urban and industrial discharges on microbial plankton was studied in Hann Bay near Dakar (Senegal) on the Atlantic Ocean. Spatial patterns were studied using three transects, with a total of 20 stations, during two periods in May and November (before and after the seasonal upwelling), revealing a clear contrast between near shore stations and the intermediate and seaward stations. In November, phytoplankton were associated with dissolved nutrient availability (e.g. with nitrate, r = 0.76) whereas in May, phytoplankton were more correlated with microbial variables (e.g. with heterotrophic nanoflagellates, r = 0.63). Most samples (40) failed to meet the quality levels for the faecal indicator bacteria (FIB) defined by the European Union bathing water quality directive. The topography plays a major role in water circulation explaining the presence of FIB at the seaward stations. The high prevalence of FIB during the two periods suggests chronic pollution and a potential risk to recreational swimmers and fish consumers in Hann Bay. Thus, as demonstrated in various temperate systems, the decline of water quality constitutes a serious problem in many West African countries.