Effects of coexistence between the blue mussel and eelgrass on sediment biogeochemistry and plant performance

Abstract

The habitat-modifying suspension-feeding mussel Mytilus edulis may have facilitating or inhibiting effects on seagrass meadows depending on the environmental conditions. We investigated the effects of M. edulis on sediment biogeochemistry in Zostera marina meadows under eutrophic conditions in Flensborg fjord, Denmark. Sediment and plant samples were collected at 5 stations with Z. marina (Eelgrass), 5 with Z. marina and M. edulis (Mixed), and at 2 unvegetated ones, 1 with mussels (Mussel) and 1 with sand (Sand). The Mixed sediment was enriched in fine particles (2 to 3 times), nutrients and sulphides compared to Eelgrass stations. Increased sediment nutrient availability at the Mixed stations was reflected in increased N and P content in eelgrass. However, the plant biomass did not differ significantly between stations, while shoot features (number of leaves and leaf area) were significantly reduced at Mixed stations, suggesting an inhibiting effect of M. edulis on Z. marina. Negative correlations between eelgrass measures and sediment sulphide at Mixed stations indicate that the presence of mussels increases sulphide invasion in the plants. A survey of 318 stations in Danish fjords suggests a threshold of 1.6 kg M. edulis m −2 beyond which no coexistence between Z. marina and M. edulis was found.

Light indirectly mediates bivalve habitat modification and impacts on seagrass

“…In the Baltic Sea, studies of the effects of M. edulis on Z. marina have had particularly conflicting results (e.g., Reusch et al, 1994;Worm and Reusch, 2000;Vinther et al, 2012). This study was conducted with seawater, sediments, and organisms collected from the Danish Straits, which connect the North Sea and Baltic Sea.…”

“…To half of the mesocosms, 28 mussels (=891 mussels m −2 or 27.6 ± 3.6 g dry soft tissue per mesocosm) were carefully added in their natural orientation, creating an unhummocked epibenthic layer of approximately 100% mussel cover. This mussel density corresponds to densities commonly found in mixed eelgrass-mussel patches in the Baltic Sea (Reusch et al, 1994;Reusch and Chapman, 1995;Vinther et al, 2012). Mussels that died during the experiment (b5%) were replaced.…”

“…This study was conducted with seawater, sediments, and organisms collected from the Danish Straits, which connect the North Sea and Baltic Sea. Eelgrass and blue mussels are widely distributed within many Danish fjords and along most Danish coastlines, co-occurring in both mixed and patchy spatial distributions (Reusch et al, 1994;Worm and Reusch, 2000;Kristensen, 2002;Frederiksen et al, 2004;Vinther et al, 2012). In this region of Denmark, coastal waters are often eutrophic (median total nitrogen = 550 μg N L −1 ; median chlorophyll a = 5.1 μg L − 1 ) and turbid (median turbidity = 10.0 mg dry L −1 ), resulting in highly-variable benthic light availability (Secchi depth range = 0.3-17.0 m) (Nielsen et al, 2002).…”

“…Next, undamaged terminal shoots (leaf length N 10 cm and rhizome length ≥ 2 cm, with 3-5 internodes, intact roots, and no lateral shoots) were selected and carefully removed of senescent tissues. Then, each mesocosm was filled with sediment (10 cm layer) and 28 shoots were transplanted in haphazard arrangement (=891 leaf shoots m −2 , corresponding to eelgrass densities typically observed in mixed eelgrass-mussel patches (Reusch et al, 1994;Vinther et al, 2012)). Transplanted eelgrass was allowed two weeks to establish under intermediate light (150-250 μmol photons s − 1 m − 2 ) and then each mesocosm was randomly assigned a light and mussel treatment.…”

“…For instance, clams and mussels may facilitate seagrass growth by increasing the availability of sediment nutrients through biodeposition of feces and pseudofeces (e.g., Reusch et al, 1994;Carroll et al, 2008). However, other studies have shown that sediment enrichment by mussels can inhibit the growth of seagrasses by increasing concentrations of toxic sulfides Vinther et al, 2012). In other instances, bivalves may have mixed (Reusch and Williams, 1998) or no (e.g., Worm and Reusch, 2000;Wagner et al, 2012) effects on seagrass growth.…”