Relationships between effects of environmental temperature and seston on growth and mortality of Mytilus edulis in a temperate northern estuary

Abstract

The growth and mortality of experimentally-rafted Mytilus edulis L. of known age at 7 locations in a northern estuary (Damariscotta River, Maine, USA) were related to environmental temperatures and to the presence or absence of various potential food sources. All particles were regarded as potential food substrates. Growth decreased appreciably at sites where water temperatures exceeded 20°C, but mussels survived a wide range of elevated temperature exposures, ranging from 0 to 149 degree-days in excess of 20°C. The maximum temperature was 25°C. Mortalities of mussels at all sites but did affect the extent of mortality, which increased abruptly in late summer, when water temperatures were declining. Differences in degree-days of exposure to elevated temperatures did not influence the timing of mortality, which occurred synchronously at all times, but did affect the extent of mortality, which ranged from 35 to 90%. The period of high mortality was preceded by a rapid decline in phytoplankton standing crop. Total particle concentrations decreased during this period, but the shift toward larger particles suggests that there was little, if any, decrease in total volume of material in the seston, at least through August. It is suggested that living phytoplankton provided the critical energy source for these mussels, and that the mortalities were caused by rapidly reduced ration at a time of metabolic stress. The role of temperature and the possible role of the gametogenic cycle are discussed. In addition, the feeding efficiency of M. edulis may have decreased during this period due to an increase in mean particle diameter. Chlorophyll was divided into a nannoplankton fraction ( 20 μm). A smaller size criterion for this distinction is proposed for future studies. The importance of nannoplankton to the primary production of this estuary and the role of nannoplankton and nannoplankton-sized particles in the diet of mussels in nature are discussed.