Structure and Energy Flow of a Mussel Population in a Georgia Salt Marsh1

Modiolus demissus Dillwyn was studied near Sapelo Island, Georgia. The size distribution of the mussels was bimodal with maximum numbers in the dry weight (body) classes 0–24 mg and 400–599 mg. The total weight of mussels smaller than 200 mg was less than 2% of the total population weight. Random samples gave an estimated population density of 7.8/m 2 for the entire inhabited marsh; the estimated organic biomass was 11.5 gm/m 2 , one third of which was body and two thirds was shell conchiolin. The population was most dense near the heads of small creeks, averaging 32 mussels/m 2 . The mussels, however, were not only more abundant in some types of marsh than in others, they were also clumped within areas of uniform marsh. Shell growth occurred throughout the year, especially in small and medium‐sized individuals, but it was more rapid during the warm months. The height‐to‐weight ratio changed with the seasons, the mussels being heaviest prior to spawning. Growth and mortality of mussels in flower pots in the marsh indicated a net annual population growth of 445 mg/m 2 (dry body weight) and a net annual mortality loss of 1200 mg/m 2 . Allowance for bias would place the steady state growth‐mortality rate necessary to just maintain the population somewhere between these two estimates. Population growth and gamete production accounted for an energy flux of about 13.9 and 2.8 kg‐cal/m 2 /yr, respectively. Respiration by the population accounted for approximately 39 kg‐cal/m 2 /yr, over two thirds of which took place in air during ebb tide. Total assimilation amounted to 56 kg‐cal/m 2 /yr.