Abstract
Two contrasting estuarine copepods, Acartia tonsa and Eurytemora carolleeae, the former a broadcast spawner and the latter a brood spawner, were fed a constant carbon-based diatom diet, but which had a variable N:P content, and the elemental composition (C, N, P) of tissue and eggs, as well as changes in the rates of grazing, excretion, egg production and viability were measured. To achieve the varied diet, the diatom Thalassiosira pseudonana was grown in continuous culture at a constant growth rate with varying P supply. Both copepods altered their chemical composition in response to the varied prey, but to different degrees. Grazing (clearance) rates increased for A. tonsa but not for E. carolleeae as prey N:P increased. Variable NH4+ excretion rates were observed between copepod species, while excretion of PO43− declined as prey N:P increased. Egg production by E. carolleeae was highest when eating high N:P prey, while that of A. tonsa showed the opposite pattern. Egg viability by A. tonsa was always greater than that of E. carolleeae. These results suggest that anthropogenically changing nutrient loads may affect the nutritional quality of food for copepods, in turn affecting their elemental stoichiometry and their reproductive success, having implications for food webs.
Highlights
Copepod growth and reproductive success have often been interpreted as a response to food quantity, with increasing rates related to increased food supply [1,2,3]
This study aimed to test the following hypotheses: (1) copepod stoichiometry and nutrient content should not vary significantly as food N:P stoichiometry varies, (2) excretion rates of P and N should vary in relation to the N:P in the food, (3) egg production rates, egg nutrient content, and egg viability should decline as P content in food decreases, and (4) A. tonsa, a broadcast spawner, should respond to declining P in food by decreasing egg production and viability, while E. carolleeae, the brood spawner, should maintain consistent egg viability egg production may decline when grazing on P-poor food
These results suggest that changing environmental nutrient ratios may affect the nutritional quality of food for copepods, thereby affecting their tissue elemental stoichiometry, excretion, and reproductive success which, in turn, have implications for nutrient release to the environment and food webs in systems undergoing nutrient change
Summary
Copepod growth and reproductive success (egg production rates and egg viability or hatching success) have often been interpreted as a response to food quantity (typically measured as carbon, C), with increasing rates related to increased food supply [1,2,3]. The nutritional value of food to a consumer to meet its physiological needs, may constrain copepod physiological processes [4,5,6,7]. Effects of food quality as measured by N content have been reported for a few copepods [5,7,12]. There have been comparatively few studies of the effect of variable N:P ratios of prey on estuarine copepods [7], but such studies have recently been undertaken on heterotrophic dinoflagellates, ciliates and rotifers [13,14,15,16]. This study was undertaken to determine the effects of altered N:P, as a measure of nutritional quality, on the physiology of two estuarine copepods, Acartia tonsa and Eurytemora carolleeae
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