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, ...
The species composition of plankton, insect and annual plant communities may vary markedly from year to year. Such interannual variability is usually thought to be driven by year-to-year variation in weather conditions. Here we examine an alternative explanation. We studied the effects of regular seasonal forcing on a multi-species predator-prey model consisting of phytoplankton and zooplankton species. The model predicts that interannual variability in species composition can easily arise without interannual variability in external conditions. Seasonal forcing increased the probability of chaos in our model communities, but squeezed these irregular species dynamics within the seasonal cycle. As a result, the population dynamics had a peculiar character. Consistent with long-term time series of natural plankton communities, seasonal variation led to a distinct seasonal succession of species, yet the species composition varied from year to year in an irregular fashion. Our results suggest that interannual variability in species composition is an intrinsic property of multi-species communities in seasonal environments.
On the tidal flats off Terschelling in the Dutch Wadden Sea, the seagrass Zostera noltii Hornem. and adult lugworms Arenicola marina L. show strictly separate distributions with an abrupt border between the 2 communities. Since seagrass and lugworms do not show different limitations regarding sediment type and emersion period, the border must be the result of other environmental factors or plant-animal interactions. Transplantation experiments on the tidal flats showed that the seagrass was quite capable of growing on the lugworm tidal flat when worms were absent However, the seagrass density and biomass were severely influenced in the presence of lugworms, most probably due to the worms' sediment-reworking activities. Lugworms quickly disappeared from the experimental sites on the seagrass tidal flat, irrespective of the actual presence of seagrass. A local survey of the seagrass area showed that funnel formation by lugworms to sufficient depth was restricted by a dense clay layer under the sediment surface, a remnant of a former salt marsh. These results indicate that an increase of lugworm densities in the Dutch Wadden Sea may have contributed to the decrease of the area covered by Z. noltii on the tidal flats off Terschelling over the last 20 yr.
Intertidal blue mussel beds are important for the functioning and community composition of coastal ecosystems. Modeling spatial dynamics of intertidal mussel beds is complicated because suitable habitat is spatially heterogeneously distributed and recruitment and loss are hard to predict. To get insight into the main determinants of dispersion, growth and loss of intertidal mussel beds, we analyzed spatial distributions and growth patterns in the German and Dutch Wadden Sea. We considered yearly distributions of adult intertidal mussel beds from 36 connected tidal basins between 1999 and 2010 and for the period 1968-1976. We found that in both periods the highest coverage of tidal flats by mussel beds occurs in the sheltered basins in the southern Wadden Sea. We used a stochastic growth model to investigate the effects of density dependence, winter temperature and storminess on changes in mussel bed coverage between 1999 and 2010. In contrast to expectation, we found no evidence that cold winters consistently induced events of synchronous population growth, nor did we find strong evidence for increased removal of adult mussel beds after stormy winter seasons. However, we did find synchronic growth within groups of proximate tidal basins and that synchrony between distant groups is mainly low or negative. Because the boundaries between synchronic groups are located near river mouths and in areas lacking suitable mussel bed habitat, we suggest that the metapopulation is under the control of larval dispersal conditions. Our study demonstrates the importance of moving from simple habitat suitability models to models that incorporate metapopulation processes to
Bivalve biomass in coastal marine areas tends to fluctuate heavily from year to year, but the relative impact of underlying processes (such as recruitment, mortality, and growth) on this variability is poorly known. The present study deals with 3 bivalve species that live on tidal flats and are readily exploitable as food for shellfish-eating birds, which have suffered in recent years from short food supply in the Wadden Sea. Long-term (~40 yr) monitoring of bivalve populations on tidal flats in the western Wadden Sea has revealed high year-to-year variability in numerical densities, whereas variability in growth rates and resulting mean individual weights-at-age was relatively low. High biomass values were found in particular when strong cohorts had arisen from successful recruitment in preceding years, whereas failing recruitment for some years in succession led to low biomass of bivalve stocks. Occasionally, temporarily enhanced mortality rates resulted in temporarily strongly reduced biomass values. Only in Macoma balthica were biomass values (at similar foregoing recruitment and mortality levels) lower in years with below-average individual weights than after years with above-average ones. In all 3 species, processes that govern numbers rather than individual weights were most decisive for biomass variability. Recruitment and mortality rates in only 3 species together explained a significant proportion of the variability of the total (multi-species) bird-accessible bivalve biomass. The low bird-food supply in recent years in the Wadden Sea appears to be due to a shift in the major structuring factors since the mid-1990s: as a consequence of the absence of cold winters in the area after 1997, annual recruitment in some major bivalve species has consistently failed and was not sufficiently compensated by better survival. A recent period of consistently enhanced mortality rates in M. balthica even aggravated the situation.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.