Epiphyte grazing

Sediment removal by wildfowl

In addition to the physical parameters of the environment, the growth and survival of Zostera plants will be affected by the activities of other organisms which co-occur with them in these biotopes. In this respect, two factors that have been cited are the grazing of epiphytes growing on Zostera leaves, and the removal of excess sediment by the activities of wildfowl.

Zostera leaves provide a substratum for the growth of epiphytic algae. Excessive algal growth can smother the Zostera plants, but this is counteracted by the grazing activities of animals such as gastropods. Epiphyte grazing may be important in maintaining the health of Zostera plants. Phillipart (1995) demonstrated experimentally that increased numbers of the gastropod Hydrobia ulvae reduced the density of epiphytes on Zostera noltii and led to enhanced growth of the eelgrass. He noted that populations of H. ulvae decreased on the tidal flats of the Dutch Wadden Sea in the early 1970s. This coincided with the appearance of very heavy epiphyte fouling in some areas and may have contributed to the later decline of Z. noltii. Similar results were obtained by Nelson (1997), studying Z. marina in Puget Sound. The gastropod Lacuna variegata was shown to be capable of reducing the epiphyte biomass to levels well below those sometimes found in the field. The Zostera plants had a healthier appearance in the presence of the snails and it was demonstrated that heavy epiphyte growth could permanently damage leaves.

As described previously, Zostera beds encourage sedimentation. The consequent accumulation of material can increase aerial exposure and the likelihood of desiccation. However, Jacobs et al. (1981) suggested that the feeding activities of overwintering wildfowl cause some of this sediment to be resuspended, and that this may play an important part in preventing excessive build-up of sediment around the Zostera plants.

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