Growth and C:N:P ratios in copepods grazing on N- or Si-limited phytoplankton blooms

Abstract

The aim of this study was to investigate how nutrient limitation in phytoplankton blooms affects growth and C:N:P ratios in marine pelagic copepods. We performed two mesocosm experiments on the Atlantic coast near Trondhjem (Norway). Si-limitation in a phytoplankton bloom was triggered by N and P additions (NP treatment) and N-limitation was triggered by N, P and Si additions (NPSi treatment). Both nutrient treatments stimulated microalgal growth and increased the biomass of the phytoplankton manifold. The initial phytoplankton community consisted of flagellates and diatoms. Throughout both experiments, community composition stayed relatively stable and diverse in the NP treatments, but in the NPSi treatments large and heavily silicified diatoms came to dominate completely. Phytoplankton C:N ratios in the Si-limited blooms were close to the Redfield ratio of 6.6 (on a molar basis), but they were higher in the Control treatment without nutrient additions (ca. 8.6) and up to 14 in heavily N-limited blooms. When phytoplankton blooms (chlorophyll a > 25 nmol l−1) had established, wild copepods were added to the mesocosms. During Si-limitation the copepod density increased by ca. 40% in one of the experiments, while the C:N ratio was 5.5–6 in the copepods. During N-limitation, the copepod density stayed stable, while the C:N ratio increased to ca. 7 in the course of the experiment. In the other experiment the copepod density decreased by ca. 25%, irrespective of nutrient treatment (C:N ratio ca. 9). The N:P ratios in the copepods varied between 16 and 22 and were not different in the NP and NPSi treatments. Our study shows that N-limitation in phytoplankton cells can increase the C:N ratio of their grazers, which has a reportedly negative effect on copepod growth and reproduction. Our study also shows that copepod populations can be regulated by seawater Si:N ratios via diatoms: at high ratios the growth from eggs/nauplii to copepodites was hampered. High Si:N ratios provide diatoms with possibilities to escape from grazing (large species, heavy silicification, excretion of secondary metabolites), leading to the accumulation of algal biomass without transfer to higher trophic levels.