Pigment ingestion and egg production rates of the calanoid copepod Temora longicornisr. implications for gut pigment loss and omnivorous feeding

Abstract

We examined the relationship between egg production rate (£) and pigment ingestion rate (/, from gut content corrected for 33% loss) for adult female Temora longicornis in Long Island Sound on 47 occasions. Linear regression of E on / [both variables expressed in mass of nitrogen (N) female day] was: £N = 0.0016 + 0.770 x /N. The slope, 0.77, is the apparent gross efficiency of egg production, equivalent to the gross growth efficiency (GGE) assuming that females partition all nitrogen for growth into egg production. Published work suggests that a GGE of 0.37 would be expected for herbivorous copepods. The discrepancy between the expected value of 0.37 and observed value of 0.77 could result from unquantified losses of gut pigment or because T.longicomis ingested a significant amount of nitrogen by feeding as a carnivore. We suggest that if T.longicomis females derive all of their nitrogen for growth by feeding on phytoplankton, and if no correction for pigment loss is employed, then the gut pigment method underestimates pigment ingestion by no more than a factor of two. There is disagreement among zooplanktologists about whether measurements of gut pigments can be used to provide a quantitative measure of feeding rates. Some laboratory experiments have shown that measurements of gut pigment content (GPC) do not yield accurate estimates of ingestion rates because of high and variable amounts of pigment destruction—losses of 75-99% have been reported (Conover et al., 1986; Lopez et al., 1988; Penry and Frost, 1991; Cary et al., 1992). On the other hand, others have shown that pigment losses range from 0 to 30% (Dagg and Walser, 1987; Ki0rboe and Tiselius, 1987; Pasternak and Drits, 1988; Peterson etal., 1990; Penry and Frost, 1991). One reason for such a broad range in results is that laboratory experiments can be influenced by a copepod's feeding pre-history and by the degree to which an animal is acclimated to experimental conditions (Penry and Frost, 1991; Mayzaud and Razouls, 1992). An associated problem is that when one attempts to evaluate the GPC method by constructing a pigment budget, inaccuracies can arise because pigments can be lost easily during an experiment. The puzzle is that when one compares ingestion rates measured by GPC to ingestion rates estimated from another independent method (such as measuring ingestion rate from the disappearance of phytoplankton cells), the GPC method does not yield underestimates of ingestion rate (Ki0rboe et al, 1985a; Durbin et al, 1990; Peterson et al, 1990). Clearly, we have a problem here: does the gut pigment method produce accurate results or not? Can the gut pigment method be used for the purpose for which it was originally proposed (Nemoto, 1968; Mackas and Bohrer, 1976), i.e. to estimate ingestion rates of zooplankton feeding on phytoplankton? In this paper, we evalu© Oxford University Press 855 W.T.Peterson and H.G.Dam ated the GPC technique by comparing ingestion estimated from GPC to ingestion estimated by measuring egg production rates of adult female copepods in Long Island Sound. Egg production is related to ingestion through gross growth efficiency (GGE) by: