Size, distribution and sediment biodeposition of prolific bivalves in small estuaries

Abstract

The growth and distribution of bivalves in estuaries is generally driven by access to food, hydrodynamic forces and sedimentary conditions that facilitate recruitment and allow persistent settlement. Factors such as site elevation or sediment properties have a significant impact, but there are few studies quantifying the relative importance of different factors in an entire estuary. We carried out an estuary-wide survey of the cockle Cerastoderma edule L. by sampling 343 sites in a small estuary, the Burry Inlet in South Wales, UK. We determined the extent to which site elevation, as an indicator for the duration of feeding, sediment properties and position inside the estuary, explained variation in the size and density of the bivalve. We also analysed the production of faeces and pseudo-faeces per cockle. The population consisted almost entirely of 1-year old cockles, the average size was 14.7 ± 2.7 mm and the average density was 56 ± 189 cockles 0.1 m−2 (mean ± SD). Altogether 37% of the variation in size was explained by two factors: elevation of site (15%) and amount of clay & silt in sediments (22%). Variation in density appeared to play no significant role in determining size. Density was significantly linked to sediment properties (32%) and position inside the estuary (4%). Sediment biodeposition, the amount of material discharged per cockle, was 0.032 ± 0.017 g dry weight, the whole population thereby producing an estimated 387 tonnes of faeces and pseudo-faeces in the entire estuary per day. The study confirmed that access to food is a principal driver of growth, but the relatively small proportion of the variation in size explained by site elevation highlights that other factors play an important role. Sediment characteristics in particular were of importance to variation in size as well as density. It seems plausible that sediment properties were a proxy for factors such as bedload movement and exposure, which are more likely causal factors for distribution patterns. However, the bivalves may also actively favour certain sediment compositions for ease of burrowing and food availability. We suggest that the role of active and passive movement of bivalves within an estuary needs further clarification, which would affect individual growth rates and spatial size distribution patterns. The considerable amount of mucus bound material discharged by cockles suggests that the contribution of prolific bivalves to habitat properties of small estuaries is under-estimated.