The effects of food stoichiometry and temperature on copepods are mediated by ontogeny

Abstract

Climate change is warming the oceans, increasing carbon dioxide partial pressure and reducing nutrient recycling from deep layers. This will affect carbon (C) and phosphorus (P) availability in the oceans, thus, altering the balance between the nutrient content of consumers and their food resource. The combined effects of food quality and temperature have been investigated for adult copepods; however, nauplii, the early developmental stages of copepods, often far outnumber adults, grow more rapidly and have a higher phosphorus body content and demand than later life stages. Consequently, ontogeny may affect how copepods respond to the combined stressors of increasing temperature and altered food stoichiometry. We conducted temperature-controlled experiments (24, 28 and 32 °C) where Parvocalanus crassirostris was fed either a P-replete or a P-limited phytoplankton food source. Reduced survival of nauplii and copepodites at the highest temperature was ameliorated when fed P-replete food. At higher temperatures, copepodite growth remained stable, but internal C:P stoichiometry diverged in the direction of phytoplankton C:P, suggesting that increased temperature affected copepodite stoichiometric homeostasis. In contrast, naupliar P content increased with temperature and naupliar growth was P limited, suggesting nauplii required additional phosphorus at higher temperatures. We conclude that resource stoichiometry plays a key role in how copepod survival and growth are impacted by temperature, and that ontogeny mediates these responses. Our results suggest that as the extent of warming oceans and phytoplankton nutrient limitation increase, copepod survival and the growth of early life stages may decline.

Keywords: Food web; Metabolism; Nauplii; Phosphorus; Warming; Zooplankton.

Fig. 1

Sampling regime schematic illustrating (1) when the nauplii (hour 0 and 72) and copepodites (hour 168) were collected for sampling and which measurements were taken, (2) the estimated number of zooplankton L⁻¹ surviving through the 168-h (7 day) incubation including the removal of approximately 1500 zooplankton L⁻¹ at hour 72 which yielded a remaining 500 zooplankton L⁻¹ in incubation bottles at the start of T4, and (3) the regimen of the complete replacement of food every 24 h

Fig. 2

Life history traits of the copepod P. crassirostris after 3 days (nauplii, left column) and 7 days (copepodites, right column) when fed P-limited (light) and P-replete (dark) phytoplankton at each of the three incubation temperatures: 25, 28, and 32 °C. All values are mean ± 1 standard error of the mean of three replicate incubation bottles (N = 3). Each panel is annotated with significant P values from two-way ANOVA. Significant post hoc interactions between food quality at specific temperatures are represented by *. a, b The percentage of copepods surviving; c, d the specific copepod grazing rate (cells of Tisochrysis lutea zooplankton mg C⁻¹ h⁻¹); e, f the specific growth rate (day⁻¹); g, h are the internal body stoichiometry of carbon:phosphorus (molar) for nauplii and copepodites, respectively