Estuarine Primary Production Research Articles

Mapping intertidal microphytobenthic biomass with very high-resolution remote sensing imagery in an estuarine system

Microphytobenthos (MPB) contributes significantly to estuarine primary production, so that quantifying its biomass is crucial for assessing their ecosystem functioning. Conventional sampling methods are labour-intensive, logistically challenging, and cannot provide a comprehensive spatial distribution map of MPB biomass. Satellite imagery has offered a feasible alternative for mapping large areas at various temporal and spatial resolutions. However, no imaging device with a spatial resolution consistent with the few square centimetres sampled in-situ has been used in the field. This makes it challenging to accurately relate field biomass measurements with remotely sensed radiometric observations. In this study, two similar multispectral sensors were mounted on an unmanned aerial vehicle (UAV) at different altitudes, as well as on a custom-built device specifically designed to acquire images at ∼1 m altitude, in order to collect very-high spatial resolution reflectance data of MPB biofilms at the Guadalquivir Estuary (Spain) mudflats. In addition, a hyperspectral spectroradiometer acquiring in-situ field reflectance was used for validation. Simultaneously, MPB samples were collected using a 2 mm depth contact corer method, which were analysed through high-performance liquid chromatography (HPLC) to measure the concentrations of major MPB pigments. To assess the relationship between the MPB pigments and different reflectance-based spectral indices, generalised linear mixed effects models (GLMMs) were used, achieving a significant positive relationship between chlorophylls and all spectral indices tested. These models were used to map microphytobenthic biomass, yielding a mean biomass in the range of 30–50 mg Chl-a m−2 in the Guadalquivir estuary during late winter. This study demonstrates the potential of low-altitude/high spatial resolution radiometric imaging as an efficient, rapid, and non-destructive addition to in-situ measurements of MPB biomass, providing exciting perspectives for the monitoring of estuarine systems on a millimetric scale of variability.

Phytoplankton Diversity, Spatial Patterns, and Photosynthetic Characteristics Under Environmental Gradients and Anthropogenic Influence in the Pearl River Estuary.

The Pearl River Estuary (PRE) is one of the world's most urbanized subtropical coastal systems. It presents a typical environmental gradient suitable for studying estuarine phytoplankton communities' dynamics and photosynthetic physiology. In September 2018, the maximum photochemical quantum yield (Fv/Fm) of phytoplankton in different salinity habitats of PRE (oceanic, estuarine, and freshwater zones) was studied, revealing a complex correlation with the environment. Fv/Fm of phytoplankton ranged from 0.16 to 0.45, with taxa in the upper Lingdingyang found to be more stressed. Community composition and structure were analyzed using 18S rRNA, accompanied by a pigment analysis utilized as a supplementary method. Nonmetric multidimensional scaling analysis indicated differences in the phytoplankton spatial distribution along the estuarine gradients. Specificity-occupancy plots identified different specialist taxa for each salinity habitat. Dinophyta and Haptophyta were the predominant taxa in oceanic areas, while Chlorophyta and Cryptophyta dominated freshwater. Bacillariophyta prevailed across all salinity gradients. Canonical correlation analysis and Mantel tests revealed that temperature, salinity, and elevated nutrient levels (i.e., NO3--N, PO43--P, and SiO32--Si) associated with anthropogenic activities significantly influenced the heterogeneity of community structure. The spatial distribution of phytoplankton, along with in situ photosynthetic characteristics, serves as a foundational basis to access estuarine primary productivity, as well as community function and ecosystem health.

Association Between Estuary Characteristics and Activities of the Critically Endangered Indo-Pacific Humpback Dolphin (Sousa chinensis)

The Indo-Pacific humpback dolphin (Sousa chinensis) has been reported to prefer estuary habitats. This study explored the environmental factors affecting a critically endangered population off the coast of Yunlin, Taiwan. We measured dolphin sighting rates and estuary characteristics affected by the watershed, including seven physical factors (watershed rainfall, watershed runoff, estuarine turbidity, pH, salinity, temperature, and dissolved oxygen) and two biological factors (estuarine net primary production and chlorophyll a concentration), at the Hsinhuwei River estuary in Taiwan. Dolphin activity was measured by sighting rate and behavioral indices for feeding and traveling between 2017 and 2018. We observed that when the maximum net production increased alongside rising temperatures in spring, both the dolphin sighting rate and foraging activity increased. This trend was maintained until heavy rainfall or increased river runoff occurred during late summer, which resulted in high turbidity in autumn and winter. Turbidity was significantly negatively correlated with dolphin activity (sighting rate and foraging). Furthermore, we found that dolphin traveling positively correlated with the chlorophyll a concentration and maximum net production factors, which could attract dolphins expecting more abundant prey fish in the estuary supported by the high primary production. This study provides empirical evidence on how estuary characteristics affected by the watershed can affect the sighting rate and behavioral activities of Indo-Pacific humpback dolphins.

Reconstructing primary production in a changing estuary: A mass balance modeling approach

AbstractEstuarine primary production (PP) is a critical rate process for understanding ecosystem function and response to environmental change. PP is fundamentally linked to estuarine eutrophication, and as such should respond to ongoing efforts to reduce nutrient inputs to estuaries globally. However, concurrent changes including warming, altered hydrology, reduced input of sediments, and emergence of harmful algal blooms (HABs) could interact with nutrient management to produce unexpected changes in PP. Despite its fundamental importance, estuarine PP is rarely measured. We reconstructed PP in the York River Estuary with a novel mass balance model based on dissolved inorganic nitrogen (DIN) for the period 1994–2018. Modeled PP compared well to previous estimates and demonstrated a long‐term increase and down‐estuary shift over the study period. This increase occurred despite reductions in discharge, flushing time, DIN loading, and DIN standing stock over the same period. Increased PP corresponded to increased water temperature, decreased turbidity and light attenuation, and increased photic depth and assimilation ratio, suggesting that phytoplankton in the York River Estuary have become more efficient at converting nutrients into biomass primarily due to a release from light limitation. The increase in PP also coincided with the increasing occurrence of late summer HABs in the lower York River Estuary, including the emergence of a second bloom‐forming dinoflagellate in 2007. Results demonstrate how changes concurrent with nutrient management could alter expected system responses and illustrate the utility of the mass balance approach for estimating critical rate processes like PP in the absence of observations.

Comparison of phytoplankton community structure in two tropical estuaries of East Coast of India

Phytoplankton community of two tropical river estuaries of the North-east coast of India was different due to dissolved nutrients concentration in those estuarine waters. The first study site was the Saptamukhi river estuary which is located in the Indian Sundarban (inside mangrove forest) and the second study site was Mahanadi estuary located in Orissa coast (adjacent to industrial and coastal fishing zone). The Saptamukhi estuary was mostly devoid of any anthropogenic influence and here the nutrient source was auto-generated. In Mahanadi estuarine water, the source of the major nutrients was industrial effluent and fishing waste. The Saptamukhi estuarine water had less dissolved phosphorus concentration but huge dissolved nitrogen available for phytoplankton uptake. A sufficient amount of dissolved silicate encouraged the diatom growth over other phytoplankton groups here. Due to the availability of the high amount of dissolved phosphorus and the low amount of dissolved nitrogen in the Mahanadi estuary the phytoplankton community was a mixed population of Bacillariophyceae, Cyanophyceae, Chlorophyceae and Dinophyceae and seasonal Chlorophycean bloom observed during the study period. In both the estuarine water primary productivity was high but the community respiration was higher and the estuaries were heterotrophic. Monsoonal runoff from land considerably changed the community in both estuarine water. Dissolved silicate concentration in both the estuarine water was sufficient for Diatom growth. Dissolve inorganic nitrogen and dissolved inorganic phosphate ratio played a major role for the community change of phytoplankton in two estuarine waters.

Hydrodynamics of river plume intrusion into an adjacent estuary: The Minho River and Ria de Vigo

Abstract Minho River intrusion into the Rias Baixas has a twofold impact on estuaries. On the one hand, freshwater intrusion can modify the water exchange with the shelf affecting the residence time. On the other hand, it can fertilize the area and promote phytoplankton blooms and increase estuarine primary production. For these reasons, it is crucial to understand the frequency, duration, and relaxation of river freshwater intrusions. The frequency of intrusion events into the rias was calculated by means of in situ data from 2006 to 2016 using the difference in salinity between the inner part of the estuary and the southern mouth (ΔS) as a proxy. Minho River intrusions, which are characterized by ΔS > 0, were detected in 8.9% of the available measurements in the Ria de Vigo, in 8.4% in the Ria de Pontevedra, and in only 4.5% in the Ria de Arousa. The FLOW module of the Delft3D model, which was previously calibrated and validated for the study area, was used to analyze the hydrodynamics of the intrusion events. The Ria de Vigo, which is the closest one to the Minho Estuary, was chosen to analyze the duration and intensity of the freshwater intrusions. Several consecutive events were detected in early 2010 (January 11th to February 8th) with ΔS values ranging from 0.7 to 2.0. Velocity and density profiles were analyzed in the middle part of the estuary, where seven freshwater intrusions matched the wind peaks favorable to intrusion events with an approximate delay of ~12 h. In general, the duration of the events was on the order of 1.5 days, although in one particular case the event lasted >3 days. The dynamic behavior of the freshwater intrusions was represented using velocity and density profiles along the main axis of Ria de Vigo. Finally, the phases of an intrusion event were characterized: initial positive estuarine circulation pattern; development of the intrusion; well-developed negative estuarine circulation; relaxation of the intrusion and recovery of the initial positive circulation.

Nutrient release from the sediments of the Great Bay Estuary, N.H. USA

Estuarine primary productivity and water quality are directly impacted by the processes that release nutrients from sediments. In this study, the release of nutrients by diffusion and sediment resuspension was investigated in the Great Bay, a temperate estuary on the northeast coast of the United States. Three sites were sampled in summer using porewater profiles and erosion chamber experiments. Porewater profiles showed a diffusive flux of ammonium and dissolved silicon (with average fluxes of 1.4 and 0.4 mmol m−2 day−1 respectively), while phosphate was not consistently released. Erosion chamber results showed that resuspension released ammonium and dissolved silicon in concentrations that increased with shear stress. The observed concentrations were greater than expected from the erosion of porewaters. This non-conservative release is consistent with desorption from resuspended particles and can be described by a linear relationship between the surface porewater concentration, amount of eroded sediment, and the non-dimensional adsorption coefficient K. For a moderate intensity resuspension event (0.22 Nm−2) the ammonium release is small relative to the daily diffusive release (erosion/diffusion = 0.6). By contrast, resuspension is important for dissolved silicon (erosion/diffusion = 8) and should be considered in estuarine silicon budgets. Scaling up the ammonium results indicates that the daily diffusive input is comparable to the daily summer riverine load of dissolved inorganic nitrogen. Benthic processes were found to return significant quantities of ammonium and dissolved silicon back into the waters of the Great Bay and likely contribute to the symptoms of eutrophication.

Vertical spatio-temporal patterns of phytoplankton due to migration behaviors in two shallow, microtidal estuaries: Influence on phytoplankton function and structure

Vertical patterns of phytoplankton biomass driven by phytoflagellate migration constitute a natural, short-term process with potentially important impacts on estuarine primary production and community composition. Although often considered well-mixed, phytoflagellate vertical migration patterns have been observed in shallow estuaries. We investigated vertical migration patterns in two shallow estuaries to determine how often and under what environmental conditions vertical migration patterns occurred, and to understand the potential impacts of vertical migration on phytoplankton composition and productivity. Vertical migration patterns were determined from circumannual records of decimeter-scale, semihourly, vertical profiles of chlorophyll fluorescence in two shallow, microtidal estuaries, the New River Estuary and Neuse River Estuary, North Carolina, USA. Observed migration patterns were compared with coincident measures of temperature, salinity, light, turbidity, nutrients, vertical stratification, and wind speed. A simple light × biomass model was used to estimate the influence of vertical migration patterns on total water column primary production. Collectively, between two sites in each estuary, diel vertical migration (DVM) patterns were detected on about half of the days. A secondary migration pattern reflecting a midday descent was also observed and was attributed to avoidance of intense midday surface irradiance. The likelihood of detectable DVM patterns increased under warmer conditions and lower incident irradiance, and at two of the stations was lessened by elevated wind speeds or reduced stratification intensity. Even weak stratification may be sufficient to reduce vertical mixing to levels that allow effective depth regulation by directed swimming. Modeled depth-integrated primary production was increased modestly (<10%) by observed migration patterns compared to a hypothetical vertically-homogenous biomass distribution. Access to optimal light levels and elevated bottom water dissolved inorganic nitrogen concentrations provides selective advantages that may explain phytoflagellate dominance in these estuaries, particularly during the warmer months, when production in surface waters of both systems is strongly N-limited.

Primary Productivity Temporal Fluctuations in a Nutrient-Rich Estuary due to Climate-Driven Events

In estuaries, climate and nutrients are the major drivers of primary productivity, since light availability, water movements, nutrient supplies, and physical stability determine the ability of primary producers to photosynthesize, grow, and survive. Therefore, in nutrient-rich estuaries, climate is expected to be the driving force of primary productivity. As global climate changes may increase the probability of some local ordinary weather events reaching extreme levels, it becomes pertinent to understand whether climate influences estuarine primary productivity patterns over the years. By means of long-term time series of phytoplankton, seagrass (Zostera noltii), and macroalgae (Gracilaria gracilis and Ulva spp.) production measurements, the influence of local weather events upon primary productivity was investigated in a temperate nutrient-rich estuary. The hypothesis proposed that estuarine primary productivity would show temporal fluctuations linked to climate-driven events and to the mitigation measures used to combat local anthropogenic pressures. Results revealed that, in the Mondego estuary, the most primary productivity is carried out by phytoplankton and that Z. noltii leaves and G. gracilis contribute equally to the daily productivity of the estuary, followed by Z. noltii belowground and finally by Ulva spp. Phytoplankton productivity seems to reach its highest rates in dry years, whereas a considerable decrease occurred during rainy periods. Regarding Z. noltii leaves, the highest productivity rates were attained in cold years. Moreover, the results do not support any relation between macroalgae productivity and climate events, but it would appear that the absence of local anthropogenic pressures increased its production rates. In what concerns primary productivity in temperate regions, small estuaries seem to have the ability to recover from singular intense weather events occurring over the years. Since recent climate change projections point out that drought periods are expected to increase and precipitation is expected to decrease, it was possible to infer that phytoplankton, in temperate estuaries, is likely to play an important role in the fixation of carbon dioxide acting as a sink of carbon, which, in turn, may increase coastal productivity. This knowledge could contribute to reduce uncertainty in future climate change projections and to enhance management strategies at a regional level, which have important environmental and socioeconomic repercussions.

Ecosystem Services Transcend Boundaries: Estuaries Provide Resource Subsidies and Influence Functional Diversity in Coastal Benthic Communities

BackgroundEstuaries are highly productive ecosystems that can export organic matter to coastal seas (the ‘outwelling hypothesis’). However the role of this food resource subsidy on coastal ecosystem functioning has not been examined.Methodology/Principal FindingsWe investigated the influence of estuarine primary production as a resource subsidy and the influence of estuaries on biodiversity and ecosystem functioning in coastal mollusk-dominated sediment communities. Stable isotope values (δ13C, δ15N) demonstrated that estuarine primary production was exported to the adjacent coast and contributed to secondary production up to 4 km from the estuary mouth. Further, isotope signatures of suspension feeding bivalves on the adjacent coast (Dosinia subrosea) closely mirrored the isotope values of the dominant bivalves inside the estuaries (Austrovenus stutchburyi), indicating utilization of similar organic matter sources. However, the food subsidies varied between estuaries; with estuarine suspended particulate organic matter (SPOM) dominant at Tairua estuary, while seagrass and fringing vegetation detritus was proportionately more important at Whangapoua estuary, with lesser contributions of estuarine SPOM. Distance from the estuary mouth and the size and density of large bivalves (Dosinia spp.) had a significant influence on the composition of biological traits in the coastal macrobenthic communities, signaling the potential influence of these spatial subsidies on ecosystem functioning.Conclusions/SignificanceOur study demonstrated that the locations where ecosystem services like productivity are generated are not necessarily where the services are utilized. Further, we identified indirect positive effects of the nutrient subsidies on biodiversity (the estuarine subsidies influenced the bivalves, which in turn affected the diversity and functional trait composition of the coastal sediment macrofaunal communities). These findings highlight the importance of integrative ecosystem-based management that maintains the connectivity of estuarine and coastal ecosystems.

Elevated ammonium concentrations from wastewater discharge depress primary productivity in the Sacramento River and the Northern San Francisco Estuary

Primary production in the Northern San Francisco Estuary (SFE) has been declining despite heavy loading of anthropogenic nutrients. The inorganic nitrogen (N) loading comes primarily from municipal wastewater treatment plant (WTP) discharge as ammonium (NH4). This study investigated the consequences for river and estuarine phytoplankton of the daily discharge of 15 metric tons NH4–N into the Sacramento River that feeds the SFE. Consistent patterns of nutrients and phytoplankton responses were observed during two 150-km transects made in spring 2009. Phytoplankton N productivity shifted from NO3 use upstream of the WTP to productivity based entirely upon NH4 downstream. Phytoplankton NH4 uptake declined downstream of the WTP as NH4 concentrations increased, suggesting NH4 inhibition. The reduced total N uptake downstream of the WTP was accompanied by a 60% decline in primary production. These findings indicate that increased anthropogenic NH4 may decrease estuarine primary production and increase export of NH4 to the coastal ocean.

Severe droughts reduce estuarine primary productivity with cascading effects on higher trophic levels

Using a 10‐yr time‐series data set, we analyzed the effects of two severe droughts on water‐quality and ecosystem processes in a temperate, eutrophic estuary (Neuse River Estuary, North Carolina). During the droughts, dissolved inorganic nitrogen concentrations were on average 46–68% lower than the long‐term mean due to reduced riverine input. Phytoplankton productivity and biomass were slightly below average for most of the estuary during a spring–autumn drought in 2002, but were dramatically lower than average throughout the estuary during an autumn–winter drought in 2007–2008. Droughts affected upper trophic levels through alteration of both habitat condition (i.e., bottom‐water dissolved oxygen levels) and food availability. Bottom‐water dissolved oxygen levels were near or slightly above average during the 2002 drought and during summer 2007. Concomitant with these modest improvements in bottom‐water oxygen condition, fish kills were greatly reduced relative to the long‐term average. Low‐oxygen bottom‐water conditions were more pronounced during summer 2008 in the latter stages of the 2007–2008 drought, and mesozooplankton abundances were eight‐fold lower in summer 2008 than during nondrought years. Below‐average mesozooplankton abundances persisted for well over 1 yr beyond cessation of the drought. Significant fish kills were observed in summer 2008 and 2009, perhaps due to the synergistic effects of hypoxia and reduced food availability. These results indicate that droughts can exert both ephemeral and prolonged multiyear influence on estuarine ecosystem processes and provide a glimpse into the future, when many regions of the world are predicted to face increased drought frequency and severity due to climate change.

River regulation alters drivers of primary productivity along a tropical river-estuary system

Worldwide, rivers continue to be dammed to supply water for humans. The resulting regulation of downstream flow impacts on biogeochemical and physical processes, potentially affecting river and estuarine productivity. Our study tested the hypothesis that primary production in the downstream freshwater reaches of a dammed river was less limited by light and nutrients relative to downstream estuarine primary production. In a tropical dryland Australian river estuary, we found that water-column primary productivity was highest at freshwater sites that had lowest light attenuation. Nitrogen may also have limited primary productivity. Below the freshwater zone was a region of macrotidal mixing with high concentrations of suspended soil particles, nutrients and chlorophyll a, and lower but variable primary productivity rates. Light controlled productivity, but the algal cells may also have been osmotically stressed due to increasing salinity. Further downstream in the estuary, primary productivity was lower than the freshwater reaches and light and nutrient availability appear to be a factor. Therefore the reduced magnitude of peak-flow events due to flow regulation, and the resulting decrease in nutrient export, is likely to be negatively impacting estuarine primary production. This has implications for future development of dams where rivers have highly seasonal flow.

Grazing effect of the invasive reef-forming polychaete Ficopomatus enigmaticus (Fauvel) on phytoplankton biomass in a SW Atlantic coastal lagoon

In this work we evaluate the potential grazing impact of the invasive reef-forming polychaete Ficopomatus enigmaticus in a SW Atlantic coastal lagoon (Mar Chiquita coastal lagoon; 37° 40' S, 57° 23' W; Argentina). This gregarious species feeds on suspended detritus and phytoplankton. Given the large area dominated by reefs, suspension feeding by this species might reduce the lagoon phytoplankton concentration and even hinder local eutrophication. To evaluate this hypothesis in situ replicated mesocosm experiments were performed in spring 2005 and, summer and winter 2006. Mesocosms enclosing reefs and without reefs were installed and grazing intensity was measured as the difference in chlorophyll a concentration and turbidity between the reef-treatment and the treatment without reefs. Reefs of F. enigmaticus decreased the mean chlorophyll a concentration, more during summer (56% decrease) than spring (25% decrease) and winter (19% decrease). Reefs also decreased water turbidity during summer (54% decrease) and spring (23% decrease). While previous studies indicate that the physical structure of these reefs alters water flow increasing water turbidity, our evidences show that their suspension-feeding activity can counteract this effect. Water turbidity was positively correlated with chlorophyll a concentration, which suggests that phytoplankton grazing by F. enigmaticus decreased light attenuation through the water column, with a potential for enhancement of benthic primary productivity. Therefore, our results suggest that grazing by this polychaete affects overall estuarine primary production as well as the relative importance of planktonic and benthic carbon sources to higher trophic levels.

Effect of sediment type on microphytobenthos vertical distribution: Modelling the productive biomass and improving ground truth measurements

Intertidal flats are frequently colonised by microphytobenthos (MPB) assemblages that form transient biofilms at the sediment surface which are responsible for large fractions of estuarine primary production. The large spatio-temporal variability in MPB biomass distribution in concert with the fact that tidal flats can cover many km 2 makes the use of remote sensing particularly useful in assessing MPB distribution. Water content, sediment type and MPB vertical migration are variables that affect the relationship between ground truth measurements and remote sensing of benthic chlorophyll. The effect of chlorophyll depth distribution (top 2 mm) on the relationship between benthic chlorophyll and several remote sensing indices (NDVI, PI, R562/R647, derivative indices and PAM fluorescence) was investigated over a 2 year sampling period at 6 sites (Tagus estuary, Portugal). Additionally, the effect of the dark adaptation time required to measure the minimum fluorescence parameter ( F 0) was also tested. Sediment type strongly affected MPB depth distribution with muddy sites showing a strong negative exponential decay in chlorophyll with distance from the surface while sandy sites had a homogenous distribution over the same scale (2 mm). Chlorophyll content (mass per unit mass, μg g − 1 ) in the top 2 mm was better correlated with remote sensing indices than concentration (mass per unit volume, mg m − 3 ), both for NDVI (0.72 vs. 0.45) and for PAM fluorescence (0.70 vs. 0.55). Separating the data by transect increased the correlation values in all situations. A fitted model of chlorophyll depth distribution showed that the effect of asymmetrical chlorophyll depth distribution was stronger on the correlations between chlorophyll concentration and NDVI than on chlorophyll content and NDVI (0.46–2 mm vs. 0.74–125 μm, muddy site) the same was valid for fluorescence (0.66–2 mm vs. 0.92–125 μm, muddy site). Dark adapting the samples for more than 5 min did not result in any significant difference in the relationship between F 0 and chlorophyll a. The residuals from the regression of chlorophyll content on NDVI were positively correlated (0.7) with the mass per unit of mass of sediment < 63 μm and negatively (− 0.6) with chlorophyll concentration, this indicates that if no correction is performed to account for chlorophyll depth distribution both units will be strongly affected by the mass of < 63 μm particles. The results demonstrate that although expressing chlorophyll a as concentration is generally a better option for ground truth measurements care should be taken to account for chlorophyll depth distribution since strong asymmetries within the sampling depth can introduce large errors.

Nutrients and Primary Production in the Estuary of the Razdol’naya River (Amur Bay, Sea of Japan)

The behavior of the basic nutrients (NO3, PO4, SiO2) was studied in the estuary of the Razdol’naya River in low and high water, the flow was 4.3 × 106 m3/day and 10.8 × 106 m3/day, respectively. It was shown that within the limits of the euphotic zone the nutrients were characterized by a pronounced nonconservative behavior caused by their removal by phytoplankton in primary production. It was determined that phytoplankton removal of nutrients occurred with ratios ΔC : ΔNO3 : ΔP : ΔSi = 105 : 18 : 1 : 37 and ΔC : ΔNO3 : ΔP : ΔSi = 93 : 11 : 1 : 29 at a respective ratio P : NO3 : Si = 1 : 22 : 140 in low water and P : NO3 : Si = 1 : 17 : 120 in high water. It was also determined that the maximum rate of nutrient removal was 4 times higher in the high water than in the low water. The maximum value of primary production of phytoplankton was 2.5–4.0 gC/m2 day. The estuary area of the Razdol’naya River was specified by rather high production. Such a rate of estuarine primary production, caused by nutrients carried out by the river, being no less than 250 t of dry weight of phytoplankton a day, can provide daily production up to 800 t of biomass in the secondary chain of the ecosystem.

Examination of coupling between primary and secondary production in a river‐dominated estuary: Apalachicola Bay, Florida, U.S.A.

To examine the influence of river‐borne organic material on estuarine communities, we conducted a dual stable isotope study in an estuary heavily influenced by alluvial runoff. Despite significant alluvial influence, secondary production in Apalachicola Bay depends more upon estuarine primary production than upon a detrital food web supported by floodplain primary production. Two simple mixing models, floodplain‐marine and floodplain‐estuarine, indicated that the upper limits for the contribution of terrestrial organic matter to estuarine consumer diets averaged 37, 25, and 27% and 20, 19, and 25% for East Bay, Cat Point, and Dry Bar, respectively. Systematic δ13C variation of consumer organisms was found for differing locations and attributed to increasing influence of terrestrial organic matter and 13C‐depleted dissolved inorganic carbon (DIC) closer to the river mouth. The δ34S data exhibited significant variation with river flow that was attributed to an admixture of terrestrial floodplain detritus with estuarine and marine organic matter. Both δ13C and δ34S isotope data demonstrated clear distinctions between benthic and water column feeding types. Our results suggest that the estuary is dependent on riverine inflows to provide floodplain detritus during the high‐flow period and dissolved nutrients for estuarine primary productivity during the low‐flow season. Any alteration of river hydrology may adversely affect estuarine secondary production, especially during the low‐flow period when the estuary is dependent on input of dissolved nutrients to maintain a high level of primary productivity.

Seasonal Changes in Microbial Processes in Estuarine and Continental Shelf Waters of the South-eastern U.S.A.

Respiratory rates of microbial plankton in five estuaries of Georgia, U.S.A. are similar at any given time of year, both within and among the estuaries, but change from 0·16±0·02μmh−1(mean±SE) in winter to 0·95±0·08μmh−1in summer, following the Arrhenius law. In short-term enrichment experiments, heterotrophs respond to added labile carbon, thus indicating a dual temperature-substrate limitation. The relatively much greater microbial metabolic rates in the marsh sediments than in estuarine water, the dominance of estuarine primary production in south-eastern U.S.A. by higher plants, and limited phytoplankton production in turbid water may explain the carbon limitation. Respiratory rates of south-eastern U.S.A. continental shelf microbial plankton are 1·1±0·14μmh−1near shore in winter and 0·95±0·08μmO2h−1in summer. In mid-shelf they are 0·7±0·11 in winter and 1·3±0·28 in summer, and at the edge of the Gulf Stream they are 0·3±0·1 in winter and 1·0±0·3 in summer. Thus, in contrast to the estuaries, microbial respiration in shelf waters is as high throughout the year as estuarine respiration in summer, showing no significant seasonal change, but respiration in the Gulf Stream changes seasonally. Although primary production on the south-eastern shelf is relatively high, especially near shore and in intrusions of North Atlantic Central Water, no correlation was found between respiratory rates of microbial plankton and concentrations of chlorophyll a. Concentrations of dissolved free amino acids in continental shelf waters are 39–87n m with no evident seasonal cycle. Compared to other estuarine-shelf systems, the south-eastern system is known not to be highly productive of higher trophic levels, i.e. fishes and shellfish. We postulate that this is a result of the trophic structure on the south-eastern U.S. continental shelf system which in turn is a consequence of the distribution of macronutrients, principally nitrogen. Most new nitrogen is from the North Atlantic Central Water rather than from terrestrial sources and, therefore, it is less well distributed over the central shelf.

Dissolved and Particulate Organic Carbon in Chesapeake Bay

We measured dissolved and particulate organic carbon (DOC and POC) in samples collected along 13 transects of the salinity gradient of Chesapeake Bay. Riverine DOC and POC end-members averaged 232±19 μM and 151±53 μM, respectively, and coastal DOC and POC end-members averaged 172±19 μM and 43±6 μM, respectively. Within the chlorophyll maximum, POC accumulated to concentrations 50–150 μM above those expected from conservative mixing and it was significantly correlated with chlorophylla, indicating phytoplankton origin. POC accumulated primarily in bottom waters in spring, and primarily in surface waters in summer. Net DOC accumulation (60–120 μM) was observed within and downstream of the chlorophyll maximum, primarily during spring and summer in both surface and bottom waters, and it also appeared to be derived from phytoplankton. In the turbidity maximum, there were also net decreases in chlorophylla (−3 μg l−1 to −22 μg l−1) and POC concentrations (−2 μM to −89 μM) and transient DOC increases (9–88 μM), primarily in summer. These occurred as freshwater plankton blooms mixed with turbid, low salinity seawater, and we attribute the observed POC and DOC changes to lysis and sedimentation of freshwater plankton. DOC accumulation in both regions of Chesapeake Bay was estimated to be greater than atmospheric or terrestrial organic carbon inputs and was equivalent to ≈10% of estuarine primary production.

A comparison of the ichthyofaunas in two permanently open eastern Cape estuaries

The Kowie and Great Fish estuaries are situated less than 30 km apart, yet they differ considerably in terms of riverine inflow, turbidity, food resources and habitat availability. The ichthyofauna of the two estuaries were sampled using plankton, seine and gill nets. A greater ichthyofaunal richness (R) was recorded in the Kowie estuary and this is attributed to the wider range of habitats and greater degree of marine influence in this system. In contrast, all three sampling gears revealed an approximate 3:1 ratio between fish abundance in the Great Fish and Kowie estuaries. The higher abundance of fishes in the Great Fish estuary is partly attributed to the large organic and nutrient inputs into this system when compared with the Kowie system, and the influence of these inputs on estuarine primary and secondary production. Individual fish species are affected differently by turbid water conditions. Indications from this study were that piscivorous fishes (e.g. Lichia amia) which rely mainly on visual foraging methods were adversely affected by the high turbidity conditions within the Great Fish estuary, whereas piscivores (e.g. Argyrosomus hololepidotus) which rely mainly on non-visual methods were unaffected. Macrobenthic predators (e.g. Pomadasys commersonnii) and detritivo-rous fish species (e.g. Mugil cephalus) also appear to be unaffected by high suspensoid levels and were usually more abundant in the Great Fish than in the Kowie estuary. The length-frequency distributions of some of the dominanl fish species occurring in both estuaries are presented.