Fecundity of marine planktonic copepods: global rates and patterns in relation to chlorophyll a, temperature and body weight
Global rates and patterns of fecundity in marine epipelagic copepods were studied as a function of temperature, body weight of the female and concentration of chlorophyll a. We divided data into 3 groups: broadcast spawners, sac spawners (including calanoids, cyclopoids and harpacticoids) and poecilostomatoids; although the latter are sac spawners, they were treated separately, but data were too sparse to examine patterns. Fecundity was positively correlated with temperature and body weight in both broadcast and sac spawners. Michaelis-Menten relationships revealed that fecundity rates are significantly related to chlorophyll a (chl a) concentration for broadcasters, but not so for sac spawners. Broadcasting copepods have a maximum fecundity (f(max)) of 47 eggs female(-1) d(-1), with a half-saturation coefficient (K-m) of 2.4 mug chl a l(-1), for a body weight of 10 mugC individual(-1), when all data are adjusted to 15degreesC. In contrast, fecundity rates in sac spawners are ca. 5 eggs female(-1) d(-1). Of the broadcaster genera examined, Centropages spp. has the highest f(max) at 71 eggs female(-1) d(-1) (data corrected to 15degreesC), and Paracalanus spp. the lowest f(max) at 25 eggs female(-1) d(-1). In the sac-spawning Pseudocalanus spp. we found a significant relationship between fecundity and chl a, with an f(max) of only 7.8 eggs female(-1) d(-1), while for Oithona spp. no significant relationship was evident. By comparing in situ with laboratory food-saturated rates we were able to assess the degree to which fecundity is food-limited in the natural environment. The degree of food limitation increases with increasing temperature in sac spawners; at low temperatures (similar to5degreesC) in situ rates are similar to laboratory food-saturated rates, but at 25degreesC rates are 23% of laboratory food-saturation values. In nature, increasing food limitation with increasing temperature may be the result of greater food requirements to balance respiration demands, i.e. decreasing net growth efficiency in warmer situations. It may also be due to lower availability of suitable food in terms of quality or quantity with increasing temperature, possibly as a result of increased dominance of smaller phytoplankton size fractions (e.g. picoplankton) in warm waters. Food limitation in the environment may be more severe than these comparisons suggest, as laboratory food-saturated fecundity rates in broadcasters may be as low as 36% of the in situ maximum rates (fmax rates).