Seaward Stations Research Articles

Gross pelagic primary production in the Ems-Dollard estuary

Anthropogenic changes to estuaries, usually highly productive systems, may result in increased turbidity levels, suppress pelagic primary production and negatively affect ecosystem functions. The Ems-Dollard estuary, enclosed between the Netherlands and Germany, is such an estuary with a long history of human induced changes. From the mid-19th century until 1992, discharges of potato starch and straw-board industry waste water, with high contents of organic matter, negatively affected oxygen concentrations and increased nutrient levels of the system. The growing awareness of the deteriorating effects of these discharges on the ecosystem invoked a large research program, BOEDE, between 1976 and 1980. The aim of the current paper was to establish the recent gross pelagic primary production, to compare the results with the late seventies data and to relate possible changes to environmental variables such as suspended matter and nutrients. To this end, six stations from the North Sea into the inner area of the Dollard were visited 39 times in 2012–2013. During each sampling cruise, a so-called PocketBox, containing a number of sensors, provided a high spatial data resolution between stations. At each station, samples were collected for carbon fixation incubations using 14C. Average chlorophyll-a concentrations as reported in this study were lower compared to 1978–1980 for the seaward stations, and higher for the most inward station in 2012 and almost similar in 2013. Light attenuation coefficients in the outer areas were lower in 2012–2013 (1.22 m−1) compared to the values reported in the period 1976–1980 (1.59 m−1), indicating somewhat less turbid conditions. For the two other stations, light attenuation coefficients in the present study were similar to the late seventies' values, for the station near the entrance of the Ems River in the Ems-Dollard area, the coefficient was substantially higher. Pelagic primary production in the 2012–2013 period was about 120 g C m−2 y−1, mainly determined by the most seaward areas. The largest differences in gross primary production in the current study compared to the 1976–1980 period were found at the seaward stations where production in the late seventies was about 60% higher. In the Dollard-area, production hardly changed since the late seventies, but also stations near the Ems-river entrance into the Ems-Dollard area showed a clear decrease. Changes at the inward stations are to be contributed to an increased turbidity; the decrease at the outer areas to a decreased nutrient level, especially dissolved phosphorus and silicate, but possibly also nitrogen.

New Insights Into the Non‐Conservative Behaviors of Nutrients Triggering Phytoplankton Blooms in the Changjiang (Yangtze) River Estuary

AbstractFour research cruises were carried out during March and July in 2015 and 2016 in the Changjiang (Yangtze) River Estuary and the adjacent shelf. Nutrient concentrations (regarded as static parameters) were measured in the surface and bottom waters collected at 86–99 stations over the course of these cruises. In addition, unfiltered seawater samples were incubated onboard for 48 hr to measure the potential change rates of nutrients (regarded as dynamic parameters). These parameters can help directly elucidate non‐conservative behaviors of nutrients in order to determine whether seawater serves as a source or a sink. Large nutrient sinks (with more negative variation rates) were consistently found at the surface during the two July cruises at the stations just along the outside edge of the turbidity maximum zone near the mouth of the river. Negative rates, although with much smaller magnitudes, were also found in most bottom water samples in July and at both the surface and bottom in March. The high net nutrient uptake rates at the surface in the summer triggered bloom events later at the seaward stations, showing that high net nutrient uptake is the cause and high chlorophyll‐a is the consequence of the bloom. Such information about biogeochemical cycling of nutrients and the mechanisms and development of bloom events occurring in large river estuarine and coastal areas could not have been obtained if these static and dynamic parameters had not been studied together.

Allochthonous Carbon--a Major Driver of Bacterioplankton Production in the Subarctic Northern Baltic Sea.

Heterotrophic bacteria are, in many aquatic systems, reliant on autochthonous organic carbon as their energy source. One exception is low-productive humic lakes, where allochthonous dissolved organic matter (ADOM) is the major driver. We hypothesized that bacterial production (BP) is similarly regulated in subarctic estuaries that receive large amounts of riverine material. BP and potential explanatory factors were measured during May–August 2011 in the subarctic Råne Estuary, northern Sweden. The highest BP was observed in spring, concomitant with the spring river-flush and the lowest rates occurred during summer when primary production (PP) peaked. PLS correlations showed that ∼60 % of the BP variation was explained by different ADOM components, measured as humic substances, dissolved organic carbon (DOC) and coloured dissolved organic matter (CDOM). On average, BP was threefold higher than PP. The bioavailability of allochthonous dissolved organic carbon (ADOC) exhibited large spatial and temporal variation; however, the average value was low, ∼2 %. Bioassay analysis showed that BP in the near-shore area was potentially carbon limited early in the season, while BP at seaward stations was more commonly limited by nitrogen-phosphorus. Nevertheless, the bioassay indicated that ADOC could contribute significantly to the in situ BP, ∼60 %. We conclude that ADOM is a regulator of BP in the studied estuary. Thus, projected climate-induced increases in river discharge suggest that BP will increase in subarctic coastal areas during the coming century.Electronic supplementary materialThe online version of this article (doi:10.1007/s00248-015-0714-4) contains supplementary material, which is available to authorized users.

Thread-drifting juvenile Mytilus spp. in continental shelf waters off Coos Bay, Oregon, USA

ABSTRACT: On 4 sample dates in summer 2007, vertically stratified zooplankton samples and oceano-graphic data were collected at 7 stations located from 0.5 to 28 km from shore near Coos Bay, Oregon,USA. Two of the sample dates (27 June and 14 August) were characterized by upwelling conditionsand the other two (3 and 18 July) by downwelling or relaxation. We identified larval and thread-drifting Mytilus californianus and M. trossulus/galloprovincialis . Thread-drifting M. californianus and M. trossulus/galloprovincialis ranged in size from 306 to 627 μm and from 305 to 832 μm, respectively.On all sample dates, larval and thread-drifting mytilids were rare in surface waters (0 to 10 m depth),abundant below these depths at the 3 inshore stations (0.7 to 4.5 km offshore) and rare or absent at alldepths at more seaward stations. Thread drifters were caught within 10s of meters of the bottom.There was no relationship between either the strength of the alongshore currents or wave energy andthe abundance of thread drifters. Thread drifters were significantly more abundant during down-welling/relaxation events than during upwelling events. The higher abundances during downwellingmay be due to favorable conditions for thread drifting within the benthic boundary; density instabilitygenerated by the downwelling circulation interacting with the cross-shelf density gradient createsvertical currents, which may aid lift-off of thread drifters from the bottom.KEY WORDS:

Sea breeze forcing of estuary turbulence and air‐water CO2 exchange

The sea breeze is often a dominant meteorological feature at the coastline, but little is known about its estuarine impacts. Measurements at an anchored catamaran and meteorological stations along the Hudson River and New York Bay estuarine system are used to illustrate some basic characteristics and impacts of the feature. The sea breeze propagates inland, arriving in phase with peak solar forcing at seaward stations, but several hours later at up‐estuary stations. Passage of the sea breeze front raises the water‐to‐air CO2 flux by 1–2 orders of magnitude, and drives turbulence comparable to spring tide levels in the upper meter of the water column, where most primary productivity occurs in this highly turbid system. Modeling and observational studies often use remotely‐measured winds to compute air‐water fluxes (e.g., momentum, CO2), and this leads to a factor of two flux error on sea breeze days during the study.

The ecological status of Karavasta Lagoon (Albania): Closing the stable door before the horse has bolted?

Karavasta is the widest and most important lagoon in Albania. This study aimed to assess the ecological quality status of the lagoon, acquire knowledge of a natural environment which might be exploited for aquaculture, and give management hints on the basis of anthropogenic impact and ecological conditions. A sampling campaign was carried out in 2008: at six stations, benthic fauna, water, and sediment parameters were considered. Statistical analyses were carried out through multivariate procedures (PCA, classification-clustering, SIMPER, RDA, DISTLM, PERMANOVA). Ecological quality was assessed through the AZTI Marine Biotic Index (AMBI), the multivariate AMBI (M-AMBI) and the Benthic Index based on Taxonomic Sufficiency (BITS). Sediment characteristics (percent organic matter, %OM; redox potential discontinuity layer depth, RPDL; particle size composition) and salinity represented contributory influences on lagoon communities. It was possible to distinguish and characterise a confined area, and benthic communities, from a marine-influenced area and its biota. The number of species was quite low when compared with other open Adriatic lagoons. The M-AMBI and BITS classifications gave quite similar results, which seemed consistent with the ecological conditions of the lagoon, that is a distinction in the ecological quality between the seaward and landward stations, with higher ecological quality (EcoQ) at the seaward stations. Given the pressures and the ecological condition of Karavasta, an intensification of aquaculture activities must be considered with caution, since the lagoon seems at significant risk of serious hypereutrophication. This situation is made worse by the limited water exchange with the marine environment due to the irregular dredging of the communication channels.

Paradigm lost? Cross-shelf distributions of intertidal invertebrate larvae are unaffected by upwelling or downwelling

We tested the hypothesis that larvae of intertidal invertebrates are swept offshore dur- ing upwelling and shoreward during downwelling. During summer 2007, vertically stratified zoo- plankton samples and oceanographic data were collected at 7 stations located from 0.7 to 27 km from shore near Coos Bay, Oregon, USA. Half the sample dates (27 June and 14 August) were character- ized by upwelling conditions (lines of constant temperature and salinity tilted upward, and a band of cold surface water was against the coast) and the other half (3 and 18 July) were characterized by downwelling or relaxation (lines of constant temperature and salinity were flat, and warm surface waters were in contact with the shore). We identified and staged larvae of Neotrypaea californiensis, Balanus glandula, B. nubilus, Chthamalus dalli, Pollicipes polymerus, and Semibalanus cariosus/B. crenatus and identified (to species or taxa level) Mytilus californianus, M. trossulus, Hiatella arctica, Dendraster excentricus, and pinnotherid and pagurid zoea. On all sample dates, all taxa and larval stages were rare in surface waters (0 to 10 m depth) and, with one exception (B. nubilus cyprids), were abundant at the 3 inshore stations (0.7 to 4.5 km offshore) and very rare or absent at seaward stations. The average distance offshore of all taxa and larval stages ranged from 0.9 to 4 km from shore and did not vary with upwelling and downwelling. Upwelling and downwelling had no effect on the cross-shelf distribution of the larvae of intertidal invertebrates; the hypothesis that upwelling carries larvae offshore and downwelling carries them back onshore was not supported.

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.

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.

Variation in food sources of the macrobenthos along a land–sea transect: a stable isotope study

To evaluate the relative importance for the macrobenthos of possible food sources, including riverine particulate organic matter, reeds, benthic microalgae, seaweeds, coastal phyto- plankton and aquaculture-derived matter, over 194 macrofaunal species were collected from 9 inter- tidal and subtidal stations along the axis of Gokasho Bay, Japan, and their isotopic compositions ana- lyzed. The δ 13 C values for terrestrial organic matter were very different from those of the consumers (>5‰), suggesting a negligible trophic role for this source. Differences in δ 15 N between seaweeds and primary consumers were small, suggesting a minor contribution of seaweeds. The δ 13 C values of the majority of consumers fell between coastal phytoplankton and benthic microalgae, allowing the calculation of the relative contribution of these microalgae to the diets of consumers. The estimated contribution ratios and the between-site comparison of the δ 13 C value for the same species along the bay axis suggest a shift of the main food source from benthic microalgae on the mudflat to coastal phytoplankton at the subtidal seaward stations. Several consumers collected from the fish-farm area had enriched δ 15 N and depleted δ 13 C values relative to samples of the same species that occurred out- side of the farm area. The δ 13 C values of the consumers at the fish farm were significantly more depleted than those at the other stations. These findings suggest that the consumers inhabiting the farming area incorporate 15 N-enriched fish meal and 13 C-reduced cereals that are major constituents

Bacterial community and some physico-chemical characteristics in a subtropical mangrove environment in Bahrain

A study of bacterial communities and some physico-chemical parameters of a subtropical mangrove habitat in the Arabian Gulf (Bahrain) was carried out in 1993–1994. Six stations at different parts of the tidal channel were selected for sampling. The mangrove habitat was found to harbor diverse bacterial communities, included among them anoxygenic phototrophs (AP), oxygenic phototrophs (OP), organotrophs (OT), total coliforms (TC), faecal coliforms (FC) and haloalkaliphiles (HA). Spatial and temporal variations in bacterial communities and environmental parameters were found. Each of the OT, AP, TC, and FC were dominant in the innermost stations (S1&S2) and gradually decreased seaward. The HA community on the other hand, was dominant at the seaward stations (S5&S6) and was most likely to have originated from the sea through the tidal flows. Both AP and OT were considered as part of the mangrove native flora, whereas TC and FC were alien and believed to have been introduced through partially treated sewage released at the upstream of the tidal channel. Closely monitoring of the mangrove water revealed succession pattern in bacterial communities. The AP community was predominant from November 1993 to March 1994, succeeded by dominance of OP from June 1994 to October 1994. Both bacterial blooms gave water a pinkish, purple, or green color, respectively. Although OT prevailed during Transitional period between AP and OP eutrophication, it remained comparatively constant (not less than 2 × 10 5 cfu/ml) through other periods. Frequent eutrophication phenomena of OP, which took place in summer and autumn, coincided with increases in water temperature, chlorophyll a, and nutrients ( NO 3 - and PO 4 - ). On the other hand, OT and AP were negatively correlated with temperature, salinity and chlorophyll a, but no specific pattern was observed in relation to NO 3 - and PO 4 - . In comparison with seawater, nutrients such as NO 3 - and PO 4 - were consistently higher in the mangrove habitat. Partially treated sewage and farm drainage canals are proposed to form additional sources of nutrients. Although, the mangrove habitat has been demonstrated to possess self-cleaning properties, data obtained suggest that anthropogenic pollution has a deleterious effect.

Spatial profiles of hsp70 proteins in Asian clam ( Potamocorbula amurensis) in northern San Francisco Bay may be linked to natural rather than anthropogenic stressors

Multi-year investigations in northern San Francisco Bay by United States Geological Survey have linked reduced condition indices in populations of Asian clam ( Potamocorbula amurensis) with elevated cadmium tissue concentrations. Our study seeks to determine whether levels of hsp70 proteins in P. amurensis can be correlated with these findings, and/or are related to histopathologic alterations and concentrations of metallothionein-like proteins. Here we present our results on stress proteins in clams collected monthly from four field stations between July 1996 and January 1998. In addition, animals were exposed in the laboratory to a range of salinities. Stress proteins were analyzed by Western blotting using monoclonal antibodies. Hsp70 protein levels in field-collected clams were significantly higher at the seaward (high salinity/low cadmium) stations (12.5, 8.1) than at the landward (low salinity/high cadmium) stations (6.1, 4.1). Laboratory studies showed that clams exposed to 0.1 ppt salinity had markedly lower hsp70 levels than clams exposed to higher salinities. In view of our previous laboratory studies showing that cadmium induces hsp70 in P. amurensis, our present results indicate that reduced hsp70 protein levels in field-collected clams may be linked to salinity effects rather than cadmium tissue concentrations.

Detrital flows through the feeding pathway of the oyster ( Crassostrea gigas ) in a tropical shallow lagoon: δ 13 C signals

The spatial relationships and linkage of the detrital flows among the water column, the sediment and the oyster Crassostrea gigas cultured in the water column were examined by using stable carbon isotopes (δ13C) in a tropical shallow lagoon from October 1996 to June 1997. The lagoon is located in southwestern Taiwan and is isolated from the sea by sand barriers except at two tidal inlets. It receives freshwater mainly from two rivers. A total of 12 stations were set up along three transect lines, each running across the lagoon from riverine to tidal inlet localities. The δ13C values of the water-column POM exhibited a marked sea–river gradient, with values depleted from a high of −21.7‰ at seaward stations to a low of −28.2‰ at riverine stations; those in the sedimentary POM (<62 μm grain size) also revealed this trend, but to a lesser extent. Oysters of two known ages, 6 months old (“old oysters”) and newly settled individuals (“young oysters”), were transplanted from one station to each of the remaining stations, while some were left at the original station. Values of δ13C in the muscle of transplanted oysters changed in parallel with the sea–river gradient of δ13C in POM (decreasing from −16.0 to −18.5‰ in old oysters and from −16.8 to −21.9‰ in young ones). The spatial sea–river gradient of the oyster's δ13C is related not only to the distance between the site that the oyster inhabits and sea or riverine environment, but also to the tidal flow pattern that surrounds its feeding place. Although the δ13C value of the sedimentary POM was correlated with that of the water-column POM, the δ13C value of the oyster tissue was significantly correlated with that of the water-column POM, but not with that of the sedimentary POM. This suggests that the oyster feeds primarily on water-column rather than sedimentary POM.

Temporal and spatial structure in the suprabenthic community of a shallow estuary (western Portugal: Mondego river estuary)

The suprabenthic fauna of the Mondego river estuary (western Portugal) was sampled monthly between June 1996 and June 1997. Quantitative samples were taken, with a suprabenthic 500-μm mesh size net, at regularly spaced stations covering the entire south arm of the estuary. The diversity of the samples and the distribution of the species were assessed. Suprabenthic communities were identified using a divisive multivariate statistical technique and species composition, density and biomass of the dominant species of each community were compared among communities. Diversity was highest in the mouth of the estuary where density and biomass were lowest. Diversity decreased upstream and was lowest in the mid and inner estuary where density and biomass reached maximal values. Suprabenthic animals, mainly the mysid Mesopodopsis slabberi, reached high densities in inner and upstream stations, whereas the more seaward stations had lower densities but a higher number of species. The spatial patterns dominated over the temporal patterns.

Seasonal and Spatial Patterns of an Epibenthic Decapod Crustacean Assemblage in North-west Atlantic Continental Shelf Waters

To examine seasonal and spatial patterns in a mobile marine assemblage, monthly samples were taken in triplicate with a 2-m beam trawl (6-mm mesh) at three separate stations (landward of the ridge, on the ridge top, and seaward of the ridge). The assemblage was of epibenthic decapod crustaceans, and was situated at a north-west Atlantic continental shelf, sandy ridge site. The assemblage was composed of nine species and was extremely variable over time and space. The sevenspine bay shrimp (Crangon septemspinosa), the Atlantic rock crab (Cancer irroratus), the spider crab (Libinia emarginata) and the lady crab (Ovalipes ocellatus) were the numerical dominants, comprising >98% of all decapods collected. Three of these species (C. septemspinosa,C. irroratus,L. emarginata) exhibited marked spatial heterogeneity in abundance, with many fewer found on the ridge top than at either of the other two stations.Ovalipes ocellatuswas not as spatially variable.Crangonshowed two clear peaks, in spring and fall, as didLibinia, but neither appeared to use the site as a nursery area.Ovalipes ocellatusandC. irroratuseach showed a single peak of very small individuals in the summer and appeared to use this site for settlement. Komolgorov-Smirnov tests, analysis of variance and cluster analysis showed much less difference in assemblage structure between the landward and seaward stations than was demonstrated between either station and the ridge top. The presence of the sand ridge had a clear impact on the abundance and distribution of local decapod crustacean populations.

Particulate and dissolved spectral absorption on the continental shelf of the southeastern United States

Visible absorption spectra of particulate and dissolved materials were characterized on the continental shelf off the southeastern United States (the South Atlantic Bight), emphasizing cross‐shelf and seasonal variability. A coastal front separates turbid coastal waters from clearer midshelf waters. Spatial and seasonal patterns were evident in absorption coefficients for phytoplankton, detritus, and colored dissolved organic matter (CDOM); spectral shape parameters for CDOM and detritus; and phytoplankton chlorophyll‐specific absorption. The magnitude of CDOM absorption reflected seasonal differences in freshwater discharge and the salinity of the midshelf waters. In the spring of 1993 (high discharge), CDOM absorption at 443 nm was &gt;10 times that of total particulate absorption between 12 and 50 km offshore (0.28–0.69 m−1 versus 0.027–0.062 m−1) and up to 10 times the CDOM absorption measured in the previous summer (low discharge). Phytoplankton chlorophyll‐specific absorption in the blue increased with distance from shore (from &lt;0.03 m2 mg−1 in inner shelf waters to ∼0.1 m2 mg−1 at the most seaward stations in summer) and, for similar chlorophyll concentrations, was higher in summer than in the winter‐spring. These spatial and seasonal patterns in phytoplankton chlorophyll‐specific absorption can be attributed to a shift in phytoplankton species composition (from predominantly diatoms inshore to a cyanobacteria‐dominated assemblage midshelf in summer), pigment packaging, and higher carotenoid:chlorophyll with distance from shore.

Chemical Characterization of Suspended Particulate Organic Matter by Pyrolysis-Gas Chromatography Coupled with Mass Spectrometry and High Performance Liquid Chromatography in the Bottom Nepheloid Layer of the Rhône Delta

During the DYPOL-6 Mission in the Rhône delta the use of a metallic frame provided water samples at three levels of the underlying water column, 2, 1, and 0·5 m above the sea bottom. These sampling levels allowed determination of the characteristics of suspended organic material at the deeper layers of the bottom nepheloid layer and, eventually, their relations with the overlying suspended particulate matter.The results obtained by the measurement of the suspended particulate organic matter, the elemental analysis, the analysis of the main classes of organic compounds by pyrolysis-gas chromatography coupled with mass spectrometry and the determination of phenolic compounds by high performance liquid chromatography (HPLC) emphasized the functional characteristics of the Rhône deltaic system.Near the bottom, the lower water levels of the benthic nepheloid layer showed some obvious biogeochemical gradients related to the location of the sampling station: the amount of pyrolysis-derived phenolic compounds, aromatic hydrocarbons and carbohydrates in the suspended material was high at the stations directly exposed to input of the Rhône River. The stations from the river mouth to offshore areas showed the influence of the Rhône inflow. But sometimes, according to the river regime and the current orientation, the influence of the Liguro-provencal current oriented East-West was noticeable even near the river mouth. In the eastern region of the surveyed area, the stations submitted to this current indicated some specific characteristics: the nepheloid layer, concentrated 1 m above the bottom, had a high particulate organic carbon content and a relatively high amount of phenolic compounds which indicated a possible seagrass (Posidonia oceanica) origin. At the more seaward stations, although the influence of the river input was always detectable, the suspended particulate matter was less abundant and essentially supplied by the local biogenic input: the lower water levels were enriched in compounds yielding acetonitrile upon pyrolysis, i.e. nitrogen-containing compounds.If the proximity of the river source and regime influenced the suspended particulate matter distribution and the geochemical characteristics of organic matter, the evolution through time of the bottom turbid flow from one station to another also had a great influence.Near the bottom, the terrestrial character is shown in the shoreward stations by high contents of pyrolysis-derived aromatic hydrocarbons and phenols as well as by lignin-derived phenolic compounds analysed by HPLC. In contrast, a marine character appeared in the seaward stations, emphasized by high values of pyrolysis-derived compounds indicative of carbohydrates and nitrogenous compounds as well as by phenolic compounds representative of phytoplankton production. However the upper water layers of the bottom nepheloid were generally more influenced by terrestrial input.Some trends emphasizing a microstratification of the water layers in the deeper levels of the bottom nepheloid, related to the existence of very thin bottom currents, or, maybe, to resuspension processes resulting from the bottom proximity, were shown by the geochemical characteristics of the three water levels analysed.

The distribution and abundance of planktonic penaeid larvae in Tudor creek, Mombasa, Kenya

The temporal distribution and abundance of penaeid prawn larvae in surface waters of Tudor creek were studied. Only late mysis and early post mysis stages of development were encountered in the creek waters with a descending gradient from the mouth station (station 1) to the innermost station 5. Larval encounter incidence (% incidence) was highest in plankton tows taken during night spring tides (59%) followed by night neap tides (45.4%), day spring tides (34.9%) and day neap tides (21.4%). This encounter incidence was more skewed towards the seaward stations (1 and 2) during the neap tides than during spring tides. Likewise, larval density (mean catch/standard 5 min. tow) decreased less steeply up the creek during spring tides than it did during neap tides. Higher catch rates were in general obtained in night samples than in day samples at each station and in each neap/spring phase. The diel cycle had a greater effect on both incidence and abundance of larvae than did the tidal cycle. Intensified larval incursion into the creek was observed between March and June.

Pelagic biological processes along a salinity gradient in the Fly delta and adjacent river plume (Papua New Guinea)

We measured planktonic processes (primary and bacterial production, respiration) and concentrations of suspended material, nutrients, chlorophyll a, floating macroparticulate matter, zooplankton and neuston in the delta and river plume of the Fly River in Papua New Guinea in July/August 1989 and February 1990. There appeared to be conservative mixing of dissolved nutrients across the salinity gradient, but there were seasonal differences in nitrate and phosphate concentrations in freshwater entering the estuary. Phosphate concentrations (>5 μM) in freshwater entering the estuary during July/August were higher than in most large tropical rivers. Concentrations of suspended solids in the estuary and plume ranged from 82 to 3312 mg l −1 in July/August and from 2 to 681 mg l −1 in February. Suspended solids were from 1.0 to 33.6% carbon, but the average for most stations was between 2 and 12%. There was an inverse relationship between per cent carbon and the suspended solid concentrations, similar to that observed in the Changjiang estuary. The mean C:N ratio for suspended solids for most stations was from 4 to 21 (mean 7.3). Mean floating macroparticulate matter biomass in the estuary was 80 mg C m −3 (2.2 mg N m −3). Mean chlorophyll a concentrations in estuary and plume stations ranged from 0.25 to 5.07 and from 0.32 to 0.73 μg l −1, respectively. Phytoplankton blooms (chlorophyll a up to 5.07 μg l −1) occurred just inside the mouth of the delta where suspended solids were ≤70 mg l −1. In February, net primary production in the oligohaline regions of the delta ranged from 22 to 94 mg C m −2 day −1, but highest production (up to 693 mg C m −2 day −1) was recorded at the most seaward stations in the delta. In the plume, in situ production ranged from 227 to 340 mg C m −2 day −1. Community respiration (0.8–36 g C m −2 day −1) across the salinity gradient was typically 1–2 orders of magnitude higher than in situ production. Bacterial production rates ranged from 0 to 128 μg C l −1 day −1 (grand mean 18.8) indicating that bacterial respiration could account for ∼2–49% of total pelagic respiration. Zooplankton biomass ranged from < 1 to 623 mg m −3 and correlated with chlorophyll a and particulate matter. The Fly differs from other large tropical rivers in that maximum in situ primary production occurs where suspended loads are ∼70 mg l −1. This is similar to the Huanghe system, and is possible because of the relatively high concentrations of nitrate and phosphate in the waters of the Fly delta. The dominance of respiration over primary production across the river plume and the quantities of allochthonous carbon exported from the river and mangrove forests in the delta indicates that benthic standing stocks and secondary production on the inner Papuan shelf are supported mainly by detritus advected from the Fly and the other large rivers entering the Gulf of Papua.

Spatial community structure of the winter hyperbenthos of the schelde estuary, The Netherlands, and the adjacent coastal waters

A first assessment of the ecological importance of the hyperbenthic fauna in coastal and estuarine systems was made in December 1988 in a pilot study (41 stations) covering the Westerschelde and Oosterschelde estuaries, and part of the neighbouring shallow coastal area, the Voordelta. Multivariate analysis revealed three major gradients which could be divided into seven spatially defined hyperbenthic communities. Canonical Correspondence Analysis was applied to correlate these species assemblages with a number of environmental variables measured at each station. Species distributions in the Westerschelde appear to be primarily determined by a gradient involving salinity, turbidity and dissolve oxygen. Hyperbenthic animals, mainly the mysid Neomysis integer, reached high densities in the brackish part (>12 ind·m −2), whereas the more seaward stations had lower densities but a higher number of species. In the benthic filter-feeder dominated Oosterschelde, the total density of the hyperbenthos was very low (<0.05 ind·m −2). The shallow coastal area had intermediate densities. There was a clear gradient from offshore to inshore but the environmental variables measured did not correlate well with this gradient. Though there were substantial overlaps between the clusters, as defined by the different multivariate techniques used, the Voordelta area can be divided into three main subareas.