Rob Dekker

Impacts of Nutrient Reduction on Coastal Communities

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 variations in limiting nutrients (phosphate and silicon) were weakly correlated with biomass and more strongly with community structures of phytoplankton, macrozoobenthos and estuarine birds. Although we cannot conclusively determine if, and if so to what extent, nutrient enrichment and subsequent nutrient reduction actually contributed to the concurrent trends in these communities, it appears likely that part of the variance in the studied coastal communities is related to changes in nutrient loads. Our results imply that nutrient reduction measures should not ignore the potential consequences for policies aimed at bird conservation and exploitation of marine living resources.

Zoobenthic biomass limited by phytoplankton abundance: evidence from parallel changes in two long-term data series in the Wadden Sea

Abstract We address the question of whether year-to-year variability in pelagic algal food supply can explain long-term variability in macrozoobenthic biomass in an estuarine area. Starting in the early 1970s, quantitative data were frequently collected in standardized ways in the western part of the Dutch Wadden Sea on (1) concentrations of phytoplankton species and chlorophyll (and rates of primary production) in the main tidal inlet (Marsdiep) and (2) numerical densities and biomass of macrozoobenthic animals (and growth rates in a few species) in a nearby extensive tidal-flat area (Balgzand). In both data series, the most distinctive feature was a sudden change that took place around 1980, viz. a rather sudden and persisting doubling of concentrations of chlorophyll and algal cells and of primary production rates, as well as of numerical densities and biomass of zoobenthos. From these parallel changes we hypothesise that algal food largely determines the abundance of zoobenthos in the Wadden Sea. The following observations substantiate this hypothesis: (1) the significant correlation between annual mean values of chlorophyll concentration and overall mean numerical density and biomass of zoobenthos (as estimated after an appropriate time lag), (2) the observed limitation of zoobenthic biomass doubling (after the doubling of food supply) to areas with already high biomass values (where food demand was high and food could therefore be in short supply), (3) the limitation of a strong response to changes in food supply to functional groups that are directly dependent on algal food, i.e. suspension and deposit feeders, as opposed to carnivores, (4) the significant correlation between annual growth rates in Macoma balthica and food supply in the growing season, particularly in areas close to the tidal inlet where food concentrations were monitored. Some other factors were identified that could decisively influence zoobenthic abundance locally and/or temporarily. Harsh environmental conditions will have limited zoobenthic biomass in extreme areas such as the upper part of the intertidal and areas exposed to strong currents and wave action. Severe winters temporarily reduced the abundance of several sensitive (southern) species, including most of the infaunal and epifaunal predators. Even stronger temporary and local reductions of zoobenthic biomass occurred as a consequence of fishery activities for such benthic species as cockles, mussels and lugworms. Recovery after temporary biomass reductions was generally rapid, but late-winter values of total-Balgzand zoobenthos biomass never exceeded an upper level of 45 g AFDW per m 2 probably set by maximal annual rates of primary production of between 400 and 500 g C per m 2 .

How predictable is high bivalve recruitment in the Wadden Sea after a severe winter

Higher than average recruitment among bivalves on the intertidal flats of the Wadden Sea was observed many times after severe winters in the period 1940 - 1995. The occurrence of another severe winter in 1995/96 prompted us to test the hypothesis of severe winters leading to universally high bivalve recruitment on a large geographic scale (500 km coastline) in temperate shallow waters. We analysed data sets on bivalve abundance from seven areas in the Dutch, German and Danish Wadden Sea. The longer data sets showed generally higher bivalve recruitment in the 1970Žs and 1980Žs than in the 1990Žs which may be related to the near absence of severe winters since 1987. Considering the period 1988 onwards (the longest possible period in which 1995/96 was the only severe winter), recruitment of Cerastoderma edule was in 1996 - in agreement with our hypothesis - above average at all seven investigated areas. In contrast, recruitment of Macoma balthica and Mya arenaria was for the same period above average only in the southern Wadden Sea (south-west of Jade Bay) but not in the northern Wadden Sea (north of Eiderstedt peninsula). These regional differences may be related to (i) the different topography of the northern Wadden Sea (with barrier islands westwards to the mainland) compared to the southern Wadden Sea (with barrier islands northwards to the mainland) and subsequent differential effects of wind induced currents on bivalve recruitment, (ii) differences in biotic factors such as standing stocks, larval supply or epibenthic predation or (iii) changes in environmental conditions. Our results demonstrate that large-scale comparisons along coasts are an indispensable addition to insights derived from local studies alone.

Long-term changes (1977–1994) in intertidal macrozoobenthos of the Dollard (Ems estuary) and effects of introduction of the North American spionid polychaeteMarenzelleria cf.wireni

Intertidal macrozoobenthos was monitored twice a year at three permanent transects in the Dollard (Ems estuary) from 1977 till 1994. In this period organic waste discharges in August-December into the Dollard decreased from 25×l06 kg BOD5 (1970s) to 10×106 kg BOD5 (early 1980s), and finally to 0.5×l06 kg BOD5 (early 1990s). Few long-term trends in numerical density or biomass of separate benthos species could be demonstrated. Crustaceans, mainly the amphipodCorophium volutator, increased in abundance and biomass. In the bivalveMacoma balthica and the polychaeteNereis diversicolor decreasing trends were observed especially in biomass. No relationships with changes in environmental conditions as caused by the decrease in organic waste load could be demonstrated.In 1983 the North American spionid polychaeteMarenzelleria cf.wireni appeared in the Dollard, to become abundant in three years time almost doubling the mean annual macrozoobenthos biomass. This development made the share of polychaetes in the mean annual biomass to increase from 24 to 58%, whereas bivalves fell from 64 to 25%. Because of absence of demonstrable interactions between the original macrobenthic species and the newcomerM. cf.wireni it is concluded that this highly productive species has occupied an empty niche. The dramatic increase of this invader species may have obscured effects of decreased organic waste input on the intertidal benthic community.

How two large-scale “experiments” illustrate the Importance of enrichment and fishery for the functioning of the Wadden Sea ecosystem

AbstractIn the recent past, two far-reaching changes have taken place in the Dutch part of the Wadden Sea:(1)a drastic increase in the late 1970s of the annual rates of primary production and of chlorophyll concentrations (restricted to the western part of the Dutch Wadden Sea and probably induced by eutrophicated fresh water sluiced from Lake IJssel), and(2)a sudden removal in 1990 of nearly all mussel and cockle beds by commercial fishery all over the Dutch part of the Wadden Sea. These two large-scale events can be regarded as large-scale “experiments”, manipulating an almost natural ecosystem over a vast geographic area. The consequences could be followed by regularly executed monitoring programmes of the phytoplankton in Marsdiep (the main tidal inlet of the western part of the Wadden Sea) and the benthic macrofauna on Balgzand (a nearby 50 km2 tidal-flat area).As a response to the substantial and rather sudden increase in their food supply around 1978, the total benthic biomass roughly doubled, though with a time lag of about two years. This doubling of zoobenthic stocks was restricted to the part of the Wadden Sea that is directly affected by water from Lake IJssel. It was further restricted to areas with relatively high zoobenthic biomass (with high food demand relative to food supply) and moderate environmental conditions (no exposure to short immersion periods or strong currents).The response to the sudden removal in 1990 of nearly all beds of the two major bivalve species was even more dramatic: concentrations of phytoplankton were unusually high in the 1990/91 winter and phytoplankton blooming started unusually early in late-winter, causing high weights and early and rapid growth in the bivalves that had remained. Mortality rates in some benthos species were extraordinarily high during the 1990–1991 winter, probably as a consequence of birds switching from the unprecedentedly scarce mussels and cockles to other prey species. Oystercatchers and eider ducks suffered abnormally high mortality and a high proportion of these birds left the Dutch Wadden Sea earlier than in other years.From both “experiments” we conclude that the Wadden Sea is a food-limited ecosystem, both in the first and in the second link of the main food chain. Only in restricted areas with extreme environmental conditions is the fauna so scarce that competition for food cannot play a significant role. In such areas, other (stressing) factors apparently inhibit the abundance of the benthic fauna and enrichment of food supply is not effective (food, therefore, not being the limiting factor there). Unrestricted fishery appears to be a greater threat to the normal functioning of the ecosystem of the Dutch Wadden Sea than mild eutrophication.

Is reproductive failure responsible for reduced recruitment of intertidalMytilus edulis L. in the western Dutch Wadden Sea

The musselMytilus edulis is an abundant bivalve in the Dutch Wadden Sea, both in intertidal areas as well as in subtidal culture plots. From mid 1980’s to late 1990’s, strong declines in mussel intertidal populations have been observed, both in terms of occupied area and in biomass. Despite the efforts since 1999 to preserve intertidal mussel beds, abundance and biomass of mussels in the Dutch Wadden Sea are at present still much lower than in the 1970’1s. In the present paper, we tested whether reproductive failure could have been the cause for the low recruitment ofM. edulis in the western Dutch Wadden Sea. Water temperature is an important factor influencing growth and reproduction in bivalves. The observed increase in mean temperature in the Dutch Wadden Sea during the last decades suggests that an effect of temperature on the population dynamics of mussels might be expected. Nevertheless, no significant relationship was found between body condition and mean winter temperature. Therefore, a decrease in body condition and, consequently, in reproductive output, due to increasing temperature, will not have been a cause for the observed low recruitment in recent years. The seasonal patterns of individual growth supported this view: most adults developed gonads during the spawning season suggesting that reproductive (gametogenic) failure was not a cause for the low recruitment. Nevertheless, reproductive investment by the mussel population studied was not optimal since complete spawning hardly occurred. In addition, the high gonadosomatic ratio at the end of the summer did not result in heavy spawning, suggesting resorption of gonadal tissue due to unfavourable environmental conditions (too high temperature and/or food limitation during summer). Reduced recruitment could be due to poor spawning but other factors that exert their influence after spawning (predation on larvae or postlarvae, lack of settlement substrate) or on the adult population (decrease in number of spawners) are also likely to be involved in the observed low recruitment during the last years.

The intertidal zoning of cockles (Cerastoderma edule) in the Wadden Sea, or why cockle fishery disturbed areas of relatively high biodiversity

Recently, Kraan et al. (ICES J Mar Sci 64:1735–1742, 2007) reported that areas in the Wadden Sea selected by mechanical cockle dredgers were of greater zoobenthic diversity than areas that remained undredged. The present paper attempts to explain this observation, focussing on the elevation-related zoning of the macrobenthic fauna on tidal flats. Dense beds of cockles Cerastoderma edule occur in a specific intermediate intertidal-height zone where more species are present (and reach their maximal abundance) than in either lower or higher zones. This coincidence of preferred intertidal zones in many species, including cockles, may cause the higher species richness (and higher total macrozoobenthic biomass) of the fisher-chosen dredging areas as compared to other nearby non-dredged tidal-flat areas.

Fuelling conditions at staging sites can mitigate Arctic warming effects in a migratory bird

Under climate warming, migratory birds should align reproduction dates with advancing plant and arthropod phenology. To arrive on the breeding grounds earlier, migrants may speed up spring migration by curtailing the time spent en route, possibly at the cost of decreased survival rates. Based on a decades-long series of observations along an entire flyway, we show that when refuelling time is limited, variation in food abundance in the spring staging area affects fitness. Bar-tailed godwits migrating from West Africa to the Siberian Arctic reduce refuelling time at their European staging site and thus maintain a close match between breeding and tundra phenology. Annual survival probability decreases with shorter refuelling times, but correlates positively with refuelling rate, which in turn is correlated with food abundance in the staging area. This chain of effects implies that conditions in the temperate zone determine the ability of godwits to cope with climate-related changes in the Arctic.Advancing phenological timing is a risk for migratory birds, particularly in the Arctic where change is most rapid. Here, the authors show that bar-tailed godwits can adjust for phenological shifts by fuelling faster at staging areas to arrive at breeding sites in time.