Effect of Manila clam Ruditapes philippinarum on burial depth of eelgrass Zostera marina seeds and sediment porosity

Abstract

Eelgrass Zostera marina is widely distributed in coastal areas of the Northern Hemisphere; however, its continuous population decline is increasingly serious. Besides exogenous stresses, there are endopathic reasons for the decline, e.g., its seedling establishment rate, which is affected by burial depth of seeds and sediment types, is extremely low. Studies have shown that eelgrass seedlings in bivalve distribution areas are easier to establish. Moreover, large-scale bottom sowing culture for bivalves affects the bulk density of sediments which affects seed germination and seedling establishment of eelgrass. This study sought to examine the relationship between sediment modification induced by Manila clams with different body size and stocking density and the burial depth of eelgrass seeds caused by clams. The porosity of surface sediments varied with Manila clam biomass and was mainly affected by clam body size. Clams with a weight of 2.56 ± 0.12 g created more porosity. Clams with a weight of 3.71 ± 0.10 g caused more porosity than clam with a weight of 7.47 ± 0.22 g did. The sand diving of Manila clams affected the distribution of eelgrass seeds in sediments at different depths. The moderately abundant and large Manila clams promoted the burial depth of eelgrass seeds in sediments, and this effect was more obvious at 1–3 cm near the burial depth of 2 cm which is conducive to germination of eelgrass seeds and seedlings establishment. Thus, based on the bioturbation of Manila clams, the efficiency of sowing methods to restore the eelgrass population can potentially be improved.