Acartia tonsa is one of several estuarine calanoid copepods (along with Pseudodiaptomus and Eurytemora) that exhibit remarkable seasonal variation in abundance in temperate and subtropical coastal waters. It is a characteristic faunal component of the shallow coastal copepod assemblages of the South Atlantic Bight (Bowman 1971), and it is among the most ubiquitous and persistent copepods in estuarine systems along the Atlantic and Gulf coasts of the United States (Bay of Fundy, Daborn and Brylinsky 1981; Narragansett Bay, Durbin and Durbin 1981; Chesapeake Bay, Brownlee and Jacobs 1987; Beaufort, NC, Fulton 1984; Florida Bay, Kleppel and others 1998; Tampa Bay, Hopkins 1977; Corpus Christi/Neuces Bay, Buskey 1993). Its abundance and ease of culture have made it the focus of numerous field and laboratory experiments encompassing a wide variety of topics, including its behaviour, feeding ecology and trophic relationships, egg production and reproductive biology, and sensitivity to pollution. A complete review of the literature is beyond the scope of this characterization.
Knott (1980) observed peak A. tonsa abundances in the North Edisto River during April, while populations in two adjacent ponds that received controlled input of water pumped from the river reached maxima later in the spring, between May and July Acartia abundance . Pond populations of this species were more seasonally variable than in the river, with monthly means ranging from below 100 indiv./m3 during the winter months to greater than 35,000 indiv./m3 in May 1975. For the more stable river population, a monthly maximum of 19,575 indiv./m3 was observed, while densities greater than 1000 indiv./m3 persisted during all the winter months except the coldest (January).
Temporal fluctuations such as those shown by A. tonsa often coincide with environmental fluctuations, and in this species they may be, under some circumstances, mediated by the production of dormant eggs which serve to ensure survival during unfavorable conditions. Low winter temperatures (2-4°C) in New England apparently result in the production of diapause resting eggs in A. tonsa in that region. However, experiments on populations from Florida indicated that egg dormancy may not commonly occur in southern localities (Grice and Marcus 1981). In fact, water temperatures at the sites sampled by Knott (1980) did not fall below 6.5°C, with minimum monthly means in January of 9-10°C. Although they do not result in the production of diapause eggs, similar temperatures in Beaufort, North Carolina, were believed by Fulton (1983) to be associated with a decline in the abundance of A. tonsa.
D. Knott, SCDNR Marine Resources Research Institute