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The intertidal area of the Sylt-Rømø Bight was divided into 8 benthic and 1 pelagic subsystems according to habitat nature and the unique biodiversity of each. A quantitative food web network was constructed for each of the subsystems. Each flow model consists of 56 living and 3 nonliving compartments and depicts the biomass of each, a balanced energy budget for each of the living components, and the flow of energy and material between all compartments. These models were analysed by means of network analysis that revealed a considerable amount of variability between them in terms of system properties such as total system throughput (TST), development capacity, ascendency and redundancy, and a number of dimensionless ratios used in comparative systems ecology. Mussel beds stood out as the most productive subsystem at 5095 mg C m-2 d-1 , with a high TST of 33 571 mg C m-2 d-1. The amount of material recycled in each of the systems ranged from a high of 28% in the muddy sand flats to a low of 2.5% in the mussel beds, while the efficiency of energy transfer in the various systems fluctuated from a low of 3.3% in the sandy shoals to a high of 15% in the mussel beds. Mussel beds are highly specialized in terms of ascendancy and average mutual information in comparison with the other subsystems, but have less resilience. Most of the systems showed ratios between 0.8 and 1.4 for the 2 mutually exclusive system attributes of ascendency and redundancy. Relative redundancy indices calculated for the pelagic and mussel bed subsystems are low, indicating less organized systems with less resistance to disturbance.

A detailed energy flow model consisting of 56 living and 3 non-living compartments was assembled for the intertidal area of the Sylt-Rømø Bight. The model depicts the biomass of each compartment, carbon flow between the components, imports and exports, as well as an energy budget for each. The food web was analysed by means of network analysis which showed that about 17% of the total daily flow through the system is recycled through a complex cycling structure consisting of 1197 cycles. The cycling network indicated that about 99% of the recycling involves 2 to 3 compartments, with sediment bacteria and particulate organic carbon (POC) participating in most instances. Input/output analyses indicated that phytoplankton production in the Bight does not satisfy the demands of filter-feeders on an annual average basis so that about 160 mg C m -2 d -1 of phytoplankton have to be imported. We compared several dimensionless system level indices, such as internalised and normalised A/DC (ascendancy/development capacity) ratios, calculated for the Bight with those of other marine and estuarine ecosystems on a global basis. These comparisons showed that energy is rather inefficiently transferred within the Bight at a mean trophic efficiency index of 2.61%, and that most of the system level indices are lower than those of other coastal ecosystems. However, higher values were obtained for flow diversity and food web connectance compared to other systems. This study has revealed the Bight to be a highly complex system whose energy pathways appear to be sensitive to external perturbations.

The main objectives of this flume study were to (1) quantify density dependent effects of the short-leaf seagrass Zostera nolti on hydrodynamics and sediment resuspension from a sandy bed, and (2) measure the erodability of 2 contrasting sediments (sandy and muddy) and the extent to which this is modified by the presence of 2 seagrass species, Z. noltii (sandy) and Z. marina (muddy). Field measurements of near-bed tidal currents, turbulence and suspended particulate matter at 2 different Z. noltii locations (low energy [sheltered] and higher energy [exposed] environments) were interpreted in the context of the flume results. Skimming flow above the high density bed of Z. noltii was accompanied by a 40% reduction in near-bed flow, but this was offset by a 2-fold increase in turbulent kinetic energy (TKE) and bed shear stress (τ 0 ). Despite this increase in τ 0 there was an increase in sediment stabilisation with increasing seagrass density (10-fold increase in critical bed shear stress for erosion [τ e ] from 0.1 [bare sediment] to 1.0 Pa at the highest shoot density). This was largely explained by the increased microphytobenthos abundance (reflected in the higher chlorophyll a and carbohydrate contents) and a lower density of the grazer and bio-destabiliser Hydrobia ulvae. In contrast, the muddy site was more easily eroded (10-fold higher), with Z. marina having little effect on sediment erodability (bare: τ e = 0.05 Pa; Z. marina: τ e = 0.07 Pa). This higher erodability was due to differences in hydrodynamics and the physical/biological properties of the sediment. KEY WORDS: Seagrass beds · Zostera noltii · Zostera marina · Hydrodynamics · Sediment resuspensionResale or republication not permitted without written consent of the publisher