Ragnhild Asmus

Mussel beds: limiting or promoting phytoplankton?

Abstract Seasonal variation of phytoplankton over an intertidal mussel bed was measured in the Wadden Sea near the island of Sylt between February 1984 and April 1985. To quantify the uptake of phytoplankton by a mussel bed, an open flow-through system, the Sylt flume (20 × 2 × 2 m), canalized the tidal water over a bed of Mytilus edulis L. Changes in the content of phytoplankton in the water passing through the flume were used to calculate phytoplankton uptake over three tidal cycles in the summer of 1986. Phytoplankton plankton biomass was reduced by 37 ± 20% between the inflow and outflow of the flame. This figure includes active filtration of mussels and sedimentation. Phytoplankton biomass was reduced by the mussel bed over the whole size range from the smallest cells of 4 μm (or a new pg C·cell −1 ) up to the largest diatoms of several hundred μm (or a new thousand pg C·cell −1 . The higher the phytoplankton concentration, the higher the uptake by the mussel bed. There was a significant positive correlation between both concentration and uptake of phytoplankton. Parallel with the uptake of phytoplankton by the mussel bed, a higher nutrient release by the mussel bed was measured. Potential primary production based on the N : C ration of 16 : 106 was estimated, assuming that the released ammonium would be taken up entirely by phytoplankton. Following this assumption, the potential primary production induced by the nutrient release of the mussel bed is higher than the uptake of phytoplankton by the mussel bed. It is also probable that mussels extract N from particulate organic material other than phytoplankton. While mussels strongly reduce phytoplankton biomass, mussel beds also have the potential to significantly promote primary production.

The importance of grazing food chain for energy flow and production in three intertidal sand bottom communities of the northern Wadden Sea

In three intertidal sand bottom communities of the “Königshafen” (Island of Sylt, North Sea), the biomass production and respiration of phytobenthos, phytoplankton, macrozoobenthos, and in situ community metabolism were measured monthly during 1980. The study sites were characterized by different communities (Nereis-Corophium-belt, seagrass-bed,Arenicola-flat) and by a high abundance of the molluscHydrobia ulvae. Benthic diatoms are the major constituents of plant biomass in theArenicola-flat. In this community, gross primary productivity amounts to 148 g C m−2 a−1. 82 % of this productivity is caused by microbenthos, whereas phytoplankton constitutes only 18 %. In the seagrass-bed, gross primary productivity amounts to 473 g C m−2 a−1. 79 % of this is generated by seagrass and its epiphytes, whereas microphytobenthos contributes 19 %. In theNereis-Corophium-belt, only microphytobenthos is important for biomass and primary productivity (gross: 152 g C m−2 a−1). Annual production of macrofauna proved to be similar in theArenicola-flat (30 g C m−2 a−1) to that in the seagrass-bed (29 g C m−2 a−1). Only one third of this amount is produced in theNereis-Corophium-belt (10 g C m−2 a−1). The main part of secondary production and animal respiration is contributed by grazingH. ulvae. In the seagrass-bed, 83 % of the energy used for production is obtained from the grazing food chain. In theArenicola-flat and theNereis-Corophium-belt, the importance of non-grazing species is greater. A synchrony of seasonal development of plant biomass and monthly secondary production was observed. In theArenicola-flat and the seagrass-bed, where density and production of macrofauna are high, a conspicuous decrease in biomass of microbenthos occurs during the warmer season, whereas in theNereis-Corophium-belt primary production causes an increase in microphytobenthic biomass in summer and autumn. Energy flow through the macrofauna amounts to 69 g C m−2 a−1 in theArenicola-flat, 85 g C m−2 a−1 in the seagrass-bed and 35 g C m−2 a−1 in theNereis-Corophium-belt. Based on the assumption that sources of food are used in proportion to their availability, 49 g C m−2 a−1 (Arenicola-flat), 72 g C m−2 a−1 (seagrass-bed) and 26 g C m−2 a−1 (Nereis-Corophium-belt) are estimated as taken up by the grazing food chain. All three subsystems are able to support the energy requirements from their own primary production and are not dependent on energy import from adjacent ecosystems.

Nutrient fluxes in intertidal communities of a South European lagoon (Ria Formosa) – similarities and differences with a northern Wadden Sea bay (Sylt-Rømø Bay)

During an annual cycle, flux rates of oxygen, nitrate, nitrite, ammonium, phosphate and silicate were measured in light and dark bell jars at three sites in Ria Formosa (Algarve, Portugal) enclosing either a natural macrophytic community (macroalgae on sand or mud, a seagrass bed of Zostera noltii) or bare sediments. The results are compared with a preceeding study in which the same bell jar technique has been applied in the Sylt-Rømø Bay of the northern Wadden Sea. Nitrate flux was mainly directed from the water column to the benthic communities in Ria Formosa, as well as in the Sylt-Rømø Bay. However, nitrate uptake was higher in the northern, more eutrophic study area. In Ria Formosa, nutrient concentrations were lower than in the Sylt-Rømø Bay possibly due to strong water exchange with Atlantic waters. High temperatures and strong insolation had a greater impact on nitrate fluxes in Ria Formosa than in the Sylt-Rømø Bay. Bioturbating macrofauna increased ammonium efflux in the Sylt- Rømø Bay while this effect was not as pronounced in the Ria Formosa study sites. Benthic phosphate uptake dominated in the Ria Formosa and was correlated to initial phosphate concentrations in incoming waters. At both study sites, oxygen and nutrient fluxes were correlated with temperature. Additionally, flux rates were strongly influenced by biotic components and levels of eutrophication. A literature survey showed that mainly in temperate regions, material fluxes increase with temperature, whereas in warmer areas, ammonium and phosphate fluxes between sediment and water were generally lower.

Structural changes in the benthic diatom community along a eutrophication gradient on a tidal flat

Abstract In the mud- and sandflat region of the outer Königshafen off List on Sylt, the effects of the outflow from a sewage treatment plant on the benthic diatom flora were investigated. The spectrum of shapes, biomass, and diversity was determined in relation to the concentrations of phosphate, silicate, and nitrogen compounds in the overlying and pore water. The biomass increased with the available quantities of nutrients, while the diversity reached a maximum at the intermediate concentrations. Every different set of nutrient concentrations is characterized by a different diatom community. Slight inputs of nutrients led to changes in the relative abundances of forms typical of the habitat. Moderate concentrations permitted the species that are normally present in winter to occur in summer as well. In the strongly eutrophic region, nutrient-loving species that are not locally present under normal conditions formed nearly monospecific populations. A relatively constant input of nutrients almost eliminated the seasonal variations. Navicula gregaria, Nitzschia sigma, and Nitzschia tryblionella proved to be tolerant of pollution, while the genera Achnanthes and Amphora were typical in the nutrient-poor regions. The nutrient budget, particularly that of the nitrogen compounds, was found to be predominant among the physical and chemical factors.

Habitat suitability of the Wadden Sea for restoration of Zostera marina beds

Abstract A conceptual model is proposed, describing potential Zostera marina habitats in the Wadden Sea, based on reported data from laboratory, mesocosm and field studies. Controlling factors in the model are dynamics, degree of desiccation, turbidity, nutrients and salinity. A distinction has been made between a higher and a lower zone of potential habitats, each suitable for different morphotypes of Z. marina. The model relates the decline of Z. marina in the Wadden Sea to increased sediment and water dynamics, turbidity, drainage of sediments (resulting in increased degree of desiccation) and total nutrient loads during the twentieth century. The upper and lower delineation of both the higher and the lower zone of potential Z. marina habitats appear to be determined by one or a combination of several of these factors. Environmental changes in one of these factors will therefore influence the borderlines of the zones. The lower zone of Z. marina will be mainly affected by increased turbidity, sediment dynamics, degree of desiccation during low tide and nutrient load. The higher zone will be affected by increases in water and sediment dynamics, desiccation rates and nutrient loads. Potential Z. marina habitats are located above approx. –0.80 m mean sea level (when turbidity remains at the same level as in the early 1990s) in sheltered, undisturbed locations, and preferably where some freshwater influence is present. At locations with a high, near-marine, salinity, the nutrient load has to be low to allow the growth of Z. marina. The sediment should retain enough water during low tide to keep the plants moist. Our results suggest that the return of Z. marina beds within a reasonable time-scale will require not only suitable habitat conditions, but also revegetation measures, as the changes in the environment resulting from the disappearance of Z. marina may impede its recovery, and the natural import of propagules will be unlikely. Furthermore, the lower zone of Z. marina may require a genotype that is no longer found in the Wadden Sea.

Phytoplankton-Mussel Bed Interactions in Intertidal Ecosystems

Dense aggregations of filter-feeding molluscs characterize the benthic fauna of many estuarine and shallow coastal regions. These regions meet the light and nutrient requirements for intensive phytoplankton growth which is the basis of food resources for bivalves. Tidal currents transport water masses from a large surrounding area to the sites where banks of bivalve filter feeders occur and enable a continuous input of fresh phytoplankton. Comparing the volume of tidal water resident on a tidal flat during a tidal cycle to the water volume filtered by the entire mussel or oyster community, it is evident that the concentration of phytoplankton is distinctly influenced by the bivalve aggregations. Both grazing on phytoplankton and fertilization of the water by bivalves has to be considered, when describing the interactions between mussel beds and phytoplankton. We will review here recent publications and experiments focussed on these bivalve filter feederphytoplankton interactions.

Material exchange and food web of seagrass beds in the Sylt-Rømø Bight: how significant are community changes at the ecosystem level?

Abstract Material exchange, biodiversity and trophic transfer within the food web were investigated in two different types of intertidal seagrass beds: a sheltered, dense Zostera marina bed and a more exposed, sparse Z. noltii bed, in the Northern Wadden Sea. Both types of Zostera beds show a seasonal development of above-ground biomass, and therefore measurements were carried out during the vegetation period in summer. The exchange of particles and nutrients between seagrass beds and the overlying water was measured directly using an in situ flume. Particle sedimentation [carbon (C), nitrogen (N) and phosphorus (P) constituents] from the water column prevailed in dense seagrass beds. In the sheltered, dense seagrass bed, a net particle uptake was found even on windy days (7–8 Beaufort). Dissolved inorganic N and orthophosphate were mainly taken up by the dense seagrass bed. At times of strong winds, nutrients were released from the benthic community to tidal waters. In a budget calculation of total N and total P, the dense seagrass beds were characterised as a material sink. The seagrass beds with sparse Z. noltii were a source of particles even during calm weather. The uptake of dissolved inorganic N in the sparse seagrass bed was low but significant, while the uptake of inorganic phosphate and silicate by seagrasses and their epiphytes was exceeded by release processes from the sediment into the overlying water. Estimates at the ecosystem level showed that material fluxes of seagrass beds in the Sylt-Rømø Bight are dominated by the dense type of Zostera beds. Therefore, seagrass beds act as a sink for particles and for dissolved inorganic nutrients. During storms, seagrass beds are distinct sources for inorganic nutrients. The total intertidal area of the Sylt-Rømø Bight could be described as a sink for particles and a source for dissolved nutrients. This balance of the material budget was estimated by either including or excluding seagrass beds. Including the subtidal part, the function of the ecosystem as a source for particles increased, supposing that all seagrass beds were lost from the area. During the vegetation period, seagrass beds act as a storage compartment for material accumulated in the living biomass of the community. There was great biodiversity among the plant and animal groups found in intertidal seagrass beds of the Sylt-Rømø Bay, representing 50–86% of the total number of species investigated, depending on the particular group. Since most species are not exclusively seagrass residents, the loss of intertidal seagrass beds would be of minor importance for biodiversity at the ecosystem level. Food web structure in seagrass beds is different from other intertidal communities. Primary production and detritus input is high, but secondary production is similar to that of unvegetated areas, although the relative importance of the trophic guilds is different. The loss of seagrass beds leads to profound alterations in the food web of the total ecosystem. Historical as well as recent changes in material fluxes and energy flow due to man-made alterations to the ecosystem are discussed.

Field measurements on seasonal variation of the activity of primary producers on a sandy tidal flat in the northern Wadden sea

Abstract In 1980 chlorophyll a and biomass of phytoplankton showed two distinct peaks in spring and late summer. Thalassiosira nordenskioldii dominated the spring bloom, whereas in late summer Thalassiosira eccentrica and Biddulphia rhombus reached a maximum. Microphytobenthos biomass was dominated by the small species Achnanthes hauckiana . Highest biomass of microphytobenthos was attained in late winter. Gross primary productivity of the community amounted to 100 g C · m −2 · a 1 during time of submersion, 68% of which produced by microphytobenthos, 32% by phytoplankton. In spite of increasing production in summer, microphytobenthos did not increase in biomass. This is assumed to be due to grazing by the snail Hydrobia ulvae .

Benthic-pelagic flux rates on mussel beds: tunnel and tidal flume methodology compared

Material flux rates in an intertidal mussel bed were measured synchronously over two tidal cycles in June 1989 with Benthic Ecosystem Tunnels and a double lane flume. The tunnels enclosed the near bottom water, whereas the flume canalized the total water column. One tunnel was set up in a mussel bed and another one in an adjacent sand bottom as a control. The flume enclosed a mussel lane and a sand lane. In the tunnel and in the flume the mussel bed revealed ammonium and phosphate discharge. At the same time, phytoplankton, dominated byPhyaeocystis globosa, was taken up intensively. These flux rates showed the same tendency but they were higher in the flume than in the tunnel. Different tendencies and flux rates for oxygen and particulate organic matter (POC, PN) were found in flume and tunnel. These differences demonstrate the importance of water column processes regarding the material exchange of a mussel bed. Tunnels enclose smaller bodies of water and are therefore expected to detect even small effects of the benthos on the passing water. In flumes, benthic influence may be diluted over the entire water column but conditions are more natural. The use of flumes is restricted to shallow waters while tunnels have the potential to be used at any depth.

Trophic relationships in tidal flat areas: To what extent are tidal flats dependent on imported food?

In four intertidal areas of ‘Konigshafen’ (island of Sylt, FRG), biomass and production of macrozoobenthos were measured monthly in 1980 and 1984. The areas were characterized by different macrofauna assemblages (Nereis-Corophium belt, seagrass bed, Arenicola flat and mussel bed). Biomass and production of macrofauna were partitioned with regard to food preference of single species as well as to the food availability within their habitat. In the Nereis-Corophium belt, seagrass bed and the Arenicola flat, most of the secondary production of the macrofauna was formed by grazing animals. Secondary production of mussel beds was nearly 10 times higher than in the other three assemblages. The suspension feeder assemblage depended on planktonic food imported from outside the bay. Considering the secondary production of the total tidal flat area, suspension feeders dominated the other trophic groups, indicating a key position of this group relative to the other macrofaunal assemblages. Mussel beds regulate the seston input to other communities situated further landward. Because of this dominance of the suspension feeder group, the energy and material flow of the total tidal flat is strongly dependent on the seston input from the coastal waters of the North Sea or from other parts of the Wadden Sea.