Canal Vergés, Paula4; Valdemarsen, Thomas Bruun4; Kristensen, Erik4; Flindt, Mogens4
1 Department of Biology, Faculty of Science, SDU2 Faculty of Science, SDU3 CLEAR, Department of Biology, Faculty of Science, SDU4 Department of Biology, Faculty of Science, SDU
A Case Study from Odense Fjord
It has been suggested that current-driven macroalge transport in shallow lagoons and estuaries may negatively impact eelgrass through increased turbidity and physical stress. Increased turbidity and lower light availability for eelgrass may result when drifting macroalgae erode surface sediment and physical damage on eelgrass can occur when macroalgae are drifting as bedload. The ballistic effect of moving macroalgae on surface sediment was tested in the field as well as in a series of annular flume experiments, where simultaneous measurements of macroalgae transport and turbidity were measured at increasing current velocity. In flume experiments with macroalgae, sediment erosion always started at lower current velocities (2-4 cm s-1) than in control experiments without macroalgae (18-27 cm s-1). When macroalgae started to move, the turbidity increased immediately from a background concentration of 7–10 mg suspended particulate matter (SPM) L-1 to 30–50 mg SPM L 1 for Ceramium sp., Ulva lactuca and Chaetomorpha linum, respectively, while the more rigid Gracilaria sp. and Fucus vesiculosus lead to much higher turbidity (50–180 mg SPM L−1). Preliminary results from Odense Fjord (Denmark) confirm these results, sinc楨桧猠 e high sediment transport (>5000 g sediment m-2 d-1) and turbidity (>120 mg SPM L-1) were measured during periods of intense macroalgae drift. Furthermore, drifting macroalgae (primarily F. vesciculosus) damaged eelgrass beds and increased mortality of seedlings. Therefore high turbidity and light limitation of seagrasses may occur as a result of drifting macroalgae even in the absence of strong current forcing. The impact of drifting macroalgae in Odense Fjord was evaluated for Odense Fjord with a 3-D hydrodynamic model with a built-in particle tracking module, and it was concluded that macroalge driven resuspension occurs during most of the eelgrass growth season and that particle tracking models can be used to simulate macroalgae transport.