Y6
.5 Spatial and Temporal DislnhutlOn Patterns
change of the macrofaunal distribution along the transect. Microbial activities in
the sediment were strongly intluenced by seasonal events such as the deposition of
phytoplankton blooms in spring and autumn, mass development of macroalgae
followed by fine-grained sediment accumulation in late summer, or by ice formation during the winter months.
The spatial distribution of the macrofauna was predominantly intluenced by the
content of mud and organic material in the sediment. In the sediments underneath
the mussel bed, the macrofauna assemblage was dominated by small and fastgrowing polychaetes (r-strategists), which were deposit-feeding. In the centre of
the mussel bed, species number, abundance and biomass of the macrofauna were
low. At a small distance to the mussel bed, the macrofauna assemblage became
increasingly rich in species, biomass and individuals. With increasing distance
from the mussel bed in the direction of the sandtlat, which was devoid of mussel
biodeposits, a reduction of macrofauna species, biomass and individual numbers
was observed. Deep-dwelling, surface-deposit-feeding and predatory polychaetes
were found here.
At the second study site, the Groninger Plate, the response of a sandflat to an
excessive input of organic matter was examined. Results showed that, for about a
fortnight, oxygen and nitrate in the sediment were used up completely by the bacterial mineralization of organic substances. This was followed by an enrichment of
reduction equivalents such as sulphides, which caused that the sediment turned
anoxic up to the surface (black spots and areas) and that, under stagnant water
conditions, anoxic waterbodies appeared on the sediment surface. The anoxic
sediments emitted methane and hydrogen sulphide. Due to these unfavourable
environmental conditions, most of the infauna disappeared.
5.2.1
Microbial and Geochemical Processes
5.2.1.1
Introduction
In the Wadden Sea, phytoplankton and microphytobenthos represent the primary
producers and the first step of the nutrient and energy cycles (Asmus et al. 1998 a,
b). A large portion of primary production enters the microbial loop. Another portion of unicellular algae serves as food for filtering or grazing molluscs, worms
and crustacea (primary consumers). The plant biomass is converted into animal
biomass and partly excreted. Especially suspension-feeders are important for benthic-pelagic coupling (Smaal & Prins 1993). Their effects on biogeochemical processes in the sediment and infaunal distribution patterns were studied in the framework of the ecosystem research project ELA W A T.
Blue mussels (Mytilus edulis L.) form dense beds on tidal tlats (Chap. 3.6.1).
Fundamental processes, which modify the sediments underneath a mussel bed and
in the surrounding area, are the deposition of biodeposits (faeces and pseudofaeces), the passive capture of fine-grained material in the heterogeneous epibenthic
structure of a mussel bed and the accumulation of POC and carbonate shells
(Chap. 3.6.1; Postma 1(88). Due to hydrodynamic effects, a part of the fine-
.5 Spatial and Temporal DislnhutlOn Patterns
change of the macrofaunal distribution along the transect. Microbial activities in
the sediment were strongly intluenced by seasonal events such as the deposition of
phytoplankton blooms in spring and autumn, mass development of macroalgae
followed by fine-grained sediment accumulation in late summer, or by ice formation during the winter months.
The spatial distribution of the macrofauna was predominantly intluenced by the
content of mud and organic material in the sediment. In the sediments underneath
the mussel bed, the macrofauna assemblage was dominated by small and fastgrowing polychaetes (r-strategists), which were deposit-feeding. In the centre of
the mussel bed, species number, abundance and biomass of the macrofauna were
low. At a small distance to the mussel bed, the macrofauna assemblage became
increasingly rich in species, biomass and individuals. With increasing distance
from the mussel bed in the direction of the sandtlat, which was devoid of mussel
biodeposits, a reduction of macrofauna species, biomass and individual numbers
was observed. Deep-dwelling, surface-deposit-feeding and predatory polychaetes
were found here.
At the second study site, the Groninger Plate, the response of a sandflat to an
excessive input of organic matter was examined. Results showed that, for about a
fortnight, oxygen and nitrate in the sediment were used up completely by the bacterial mineralization of organic substances. This was followed by an enrichment of
reduction equivalents such as sulphides, which caused that the sediment turned
anoxic up to the surface (black spots and areas) and that, under stagnant water
conditions, anoxic waterbodies appeared on the sediment surface. The anoxic
sediments emitted methane and hydrogen sulphide. Due to these unfavourable
environmental conditions, most of the infauna disappeared.
5.2.1
Microbial and Geochemical Processes
5.2.1.1
Introduction
In the Wadden Sea, phytoplankton and microphytobenthos represent the primary
producers and the first step of the nutrient and energy cycles (Asmus et al. 1998 a,
b). A large portion of primary production enters the microbial loop. Another portion of unicellular algae serves as food for filtering or grazing molluscs, worms
and crustacea (primary consumers). The plant biomass is converted into animal
biomass and partly excreted. Especially suspension-feeders are important for benthic-pelagic coupling (Smaal & Prins 1993). Their effects on biogeochemical processes in the sediment and infaunal distribution patterns were studied in the framework of the ecosystem research project ELA W A T.
Blue mussels (Mytilus edulis L.) form dense beds on tidal tlats (Chap. 3.6.1).
Fundamental processes, which modify the sediments underneath a mussel bed and
in the surrounding area, are the deposition of biodeposits (faeces and pseudofaeces), the passive capture of fine-grained material in the heterogeneous epibenthic
structure of a mussel bed and the accumulation of POC and carbonate shells
(Chap. 3.6.1; Postma 1(88). Due to hydrodynamic effects, a part of the fine-