Harald 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.

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.

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.

The role of macrobenthic communities for sediment-water material exchange in the Sylt-Rømø tidal basin

The role of dominant macrobenthic communities for the material budget of the Sylt-Rømø tidal basin was investigated in the course of the SWAP project by measuringin situ flux rates with the flume technique. Mussel beds, seagrass beds andArenicola flats were found to be sinks for particles, whereas sandy shoals and elevated sands, poor in macrofauna, acted as sources for particles. Dissolved inorganic nitrogen was released by communities which were dominated by macrozoobenthos. Seagrass beds were sinks for nutrients on calm days, but released also dissolved material when storms induced high water turbulence. The total intertidal area of the basin was a sink for particles and a source for dissolved substances. However, the character of the whole ecosystem was determined by the dominating subtidal area which covered 2/3 of the area of the basin. Therefore, the subtidal exchange processes were assessed by extrapolating exchange rates from intertidal communities with comparable sediments and macrobenthic densities. Subtidal benthic communities were sparsely populated compared to the intertidal communities and their material exchange was directed from the bottom to the water column. These release processes were supported by physical forces as strong currents and water turbulence. Mussel beds were counteracting the continuous export of material from the subtidal sea floor to the water. Mussel beds and seagrass beds play an important role for the material budget, which is shown in a scenario in which both communities were excluded from the system. For the Sylt-Rømø tidal basin historical changes of benthic communities and water dynamics are reported and a possible interplay between the functioning of benthic communities and physical forces is discussed.KurzfassungDie Rolle der dominanten Makrobenthos-Gemeinschaften für den Materialaustausch des Sylt-Rømø-Tidebeckens wurde im Verlauf des SWAP-Projektes anhand von Messungen derin situ-Flußraten mit Hilfe der Flume-Technik untersucht. Für Partikel waren Muschelbänke, Seegraswiesen undArenicola-Sandwatten Senken, während Stromsände und hochgelegene Sände, die arm an Makrofauna waren, als Partikel-Quellen wirkten. Gelöster anorganischer Stickstoff wurde von solchen Gemeinschaften freigesetzt, in denen Makrozoobenthos dominierte. Seegraswiesen waren Senken für Nährsalze während ruhiger Wetterlagen, setzten jedoch Nährstoffe frei, wenn die Wasserturbulenz durch Stürme erhöht war. Das gesamte Gezeitengebiet der Bucht wirkte als Partikelsenke und als Nährsalzquelle. Trotzdem wurde das gesamte Ökosystem durch das Sublitoral geprägt, das 2/3 der Fläche der Bucht einnahm. Daher wurden für das Sublitoral Austauschraten geschätzt, indem die Austauschraten von im Sediment und der Makrobenthosdichte vergleichbaren eulitoralen Gemeinschaften auf das Sublitoral extrapoliert wurden. Die sublitoralen Benthos-gemeinschaften waren im Vergleich zu eulitoralen Gemeinschaften dünn besiedelt, und der Materialaustausch war in diesen Bereichen vom Boden in die Wassersäule gerichtet. Diese Freisetzungsprozesse wurden durch physikalische Kräfte, wie starke Strömungen, unterstützt. Muschelbänke wirkten dem kontinuierlichen Materialexport vom sublitoralen Meeresboden ins Wasser entgegen. Muschelbänke und Seegraswiesen spielten daher eine wichtige Rolle für den Stoffhaushalt. Dies zeigen Szenarios, in denen Muschelbänke oder Seegraswiesen aus dem Stoffhaushalt des Ökosystems der Sylt-Rømø-Bucht ausgeschlossen wurden. Für die Sylt-Rømø-Bucht sind historische Veränderungen sowohl in der Zusammensetzung der benthischen Gemeinschaften als auch für die Wassermenge und -dynamik bekannt. Mögliche Wechselwirkungen zwischen der Funktion der Benthosgemeinschaften und den physikalischen Kräften werden diskutiert.

Significance of Suspension-Feeder Systems on Different Spatial Scales

The role of suspension—feeders varies according to the spatial and temporal scales at which assessments are made. In this paper, biodiversity, productivity, and filtration capacity are used to assess the influences of suspension-feeders at scales ranging from individual mussels to biogeographic regions. Quantitative comparisons based on the ‘catchment areas’ of single mussels, mussel beds and entire bays are used to illustrate how the role of these organisms varies as a function of the unit of measurement. One key factor influencing the relative importance of suspension-feeders at different scales is the rate of water movement and, thus, the volume of water available to the consumers. The biodiversity within suspension-feeder guilds is important because of the way it can affect the amount and sizes of particles removed from the spectrum of available food items. Variations in the role of these animals are also observed at time scales. The complexity of the scaling problem is illustrated using examples from the suspension-feeder guild in a tidal basin of the Wadden Sea (North Sea) where experiments and field measurements have provided insights into processes and mechanisms accounting for spatial and temporal variations.

Austauschprozesse im Sylt-Rømø Wattenmeer: Zusammenschau und Ausblick

Die Engfuhrung der Wattenmeerbucht durch die sukzessiven Eindeichungen und durch die beiden Damme zu den Inseln konnten die Ursache fur eine in diesem Jahrhundert verstarkt aufgetretene Erosionstendenz sein. Der Flachenanteil der Gezeitenwatten ist von rund zwei auf ein Drittel zuruckgegangen. Auch die Deposition der feinkornigen Partikel ist vergleichsweise gering. Die biologischen Austauschprozesse werden auf drei Skalen betrachtet. Im Mikrobereich ist im Sediment die Kopplung der Funktionen eng, die Leistung hoch und der Export niedrig. Die Organismen sind klein und die Biodiversitat ist gros. Der Makroaustausch zwischen Sediment und Gezeitenwasser variiert mit der Dominanz einiger weniger Organismenarten und mit den hydrodynamischen Kraften. Die Kopplungen sind locker. Der Megaaustausch zwischen Wattenmeer und Nordsee ist noch variabler und unbestimmter. Medianwerte von Import- und Exportraten sind langfristig nicht signifikant verschieden. Einer biogenen Akkumulation wirken die Erosion und die verstarkten hydrodynamischen Krafte in dieser durch Deiche und Damme eingeengten Bucht entgegen. Im Ubergangsbereich zwischen Land und Meer drangen die physikalischen Prozesse daher allmahlich die biologische Komponente zuruck. Diese Entwicklung konnte durch neue Uberflutungsgebiete im eingedeichten, fruheren Vida Astuar etwas aufgefangen werden.