Shipboard studies were performed for testing the classical hypothesis that Antarctic phytoplankton suffers from a deficiency of Fe In a suite of 5 experiments over 8 to 12 d periods and encompassing different water masses (Weddell Sea water proper, Weddell-Scotia Confluence water, Scotia Sea water), and various plankton communities, biomass and dynamic spring/summer (ice) conditions, we always observed Fe to stimulate chlorophyll a synthesis and nutrient assimilation. In 3 out of 5 experiments there was an immediate response to added Fe, while in the other 2 expenments an effect was observed after 3 to 6 d. In 4 out of 5 experiments final particulate organic carbon (POC) levels were also higher in Fe-enriched cultures compared to controls. However the controls were also found to outgrow steadily typical chlorophyll a and POC levels found in ambient waters. This strongly suggests that the in situ Fe concentration in itself does not hamper build-up of high biomass levels. Extrapolation to the in situ ecosystem therefore suggests that, despite enhancement of phytoplankton growth, Fe is not the major factor controlling phytoplankton in the Weddell/Scotia Seas. Marginal sediments appear to supply adequate dissolved Fe for supporting at least minimum growth of phytoplankton. More remote sectors of the Southern Ocean might be more likely candidates for occasional limitation by Fe alone.
Population dynamics of common intertidal bivalves (Cerastoderma edule, Macoma balthica, Mya arenaria, Mytilus edulis) are strongly related to seawater temperatures. In northwestern European estuaries, series of mild winters followed by low bivalve recruit densities lead to small adult stocks. In this study, we examine temperature-induced effects on reproductive output (eggs m Ϫ2 ), onset of spawning (day of the year), and the juvenile instantaneous mortality rate (per day) of M. balthica. Data analysis was based on an extensive long-term data set originating from the western Wadden Sea. Our results strongly suggest that rising seawater temperatures affect recruitment by a decrease in reproductive output and by spring advancement of bivalve spawning. Apparently, global warming upsets the evolved reproductive strategy of this marine bivalve to tune its reproduction to the most optimal environmental conditions for the first vulnerable life stages, most importantly the match/mismatch of time of spawning with that of the phytoplankton bloom and the settlement of juvenile shrimps on the tidal flats. It is hypothesized that the observed density-dependent mortality of juvenile bivalves may act via competition for food, a behavioral response of shrimp to low spat densities, or be the result of the response of age and size at metamorphosis of marine bivalves to resource variability. It is to be expected that prolonged periods of lowered bivalve recruitment and stocks will lead to a reformulation of estuarine food webs and possibly a reduction of the resilience of the system to additional disturbances, such as the depletion and disturbance by shellfish fisheries.
Eutrophication due to high anthropogenic nutrient loading has greatly impacted ecological processes in marine coastal waters and, therefore, much effort has been put into reducing nitrogen and phosphorus discharges into European and North-American waters. Nutrient enrichment usually resulted in increase of biomass and production of phytoplankton and microphytobenthos, often coinciding with shifts in species composition within the primary producer community. Consequences of increasing eutrophication for higher trophic levels are still being disputed, and even less is known about the consequences of nutrient reduction on coastal food webs. Here, we present 30-year concurrent field observations on phytoplankton, macrozoobenthos and estuarine birds in the Dutch Wadden Sea, which has been subject to decades of nutrient enrichment and subsequent nutrient reduction. We demonstrate that long-term varia-
In the eutrophic Marsdiep, the westernmost tidal inlet of the Wadden Sea, phytoplankton biomass, and production almost doubled at the end of the 1970s and remained high ever since. Principal component analysis of 21-yr (1974-1994) high-resolution time series of the 32 most numerous marine algal species revealed that the phytoplankton community changed drastically both between 1976 and 1978 and again between 1987 and 1988, and that it was relatively stable in-between (1974-1976, 1978-1987) and thereafter (1988-1994). These major changes in phytoplankton biomass and species composition coincided with changes in absolute and relative (TN : TP) nutrient concentrations. During the summer of 1977, the Marsdiep shifted from a rich, but phosphorus-controlled system to an even more eutrophic but nitrogen-controlled environment. The system reshifted towards P-control between 1987 and 1988. The coincidence of the shifts in relative nutrient concentrations and phytoplankton species composition implies a strong causal relationship between TN : TP ratios and phytoplankton community structure. Among diatoms, the observed increase in phytoplankton biomass under eutrophic N-controlled conditions was particularly due to an increase of the abundance of larger algae. Our results indicate that the N budget of the area is correlated with the community structure, suggesting enhanced loss of nitrogen to the sediment through increased deposition of larger algal cells.In shallow coastal marine waters, the total primary production and biomass of phytoplankton is generally assumed to be hyperbolically related to nutrient loadings from land and subsequent availability of these nutrients in the water column (e.g., Borum and Sand-Jensen 1996). However, understanding the effect of nutrient enrichment on living resources requires detailed knowledge of how nutrients enter and leave these waters. Dissolved and particulate materials as well as living organisms are exchanged between the coastal waters and the open sea, and net material fluxes appear to depend on physical and biological responses within these systems to changes in nutrient loadings from land (e.g., Dame and Allen 1996). Phytoplankton species obviously respond in different ways to nutrient enrichment, most probably depending on their specific life-history characteristics such as growth curves and storage capacities (Grover 1997;Roelke et al. 1997). Current theories on plankton ecology predict that an eutrophic and nitrogen-controlled environment gives rise to
AcknowledgmentsWe thank J. Hegeman for his help in collecting the phytoplankton samples for more than 20 years, I. Loos for digitizing the phytoplankton data, M. Rademaker for her advice in applying different methods for estimating carbon contents of phytoplankton cells, U. Tillmann for his kindness to provide his unpublished formulae on phytoplankton cell volumes and carbon contents, and P. V. M. Bot and J. Doekes of the Dutch Ministry of Transport and Public Works for kindly supplying the nutrient data. Furthermore, ...
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