# Optimal phenology of life history events in Calanus finmarchicus: exit from diapause in relation to interannual variation in spring bloom timing and predation

## TL;DR

This study explores how copepods like Calanus finmarchicus time their exit from diapause in relation to spring blooms and predation, affecting carbon sequestration in the ocean.

## Contribution

The study introduces an individual-based model to analyze optimal diapause exit strategies in copepods under environmental variability.

## Key findings

- Copepods exit diapause earlier when predation is low, enhancing survival of early developmental stages.
- Phenotypic variance in diapause exit helps copepods adapt to interannual variability in bloom timing.
- Bet-hedging strategies are adopted to maintain population numbers during late-bloom years.

## Abstract

Respiration of lipids by copepods during diapause (overwintering dormancy) contributes to ocean carbon sequestration via the seasonal lipid pump (SLP). Parameterizing this flux in predictive models requires a mechanistic understanding of how life history adaptation in copepods shapes their timing of exit from diapause. We investigate the optimal phenology of Calanus finmarchicus in the Norwegian Sea using an individual-based model in which diapause exit is represented as a trait characterized by phenotypic mean and variance. Without interannual variability, optimal exit correlated with the onset of the spring phytoplankton bloom and phenotypic variance was of no benefit. In contrast, copepods endured reduced fitness and adopted bet-hedging strategies when exposed to interannual variability in bloom timing and predation: later exit from diapause and phenotypic variance maintained adult numbers in anomalous late-bloom years. Exit nevertheless remained well before the peak of the bloom which is a favorable strategy when low predation early in the year enhances survival of eggs and early developmental stages. Our work highlights the complex interactions between C. finmarchicus and its environment and the need for improved understanding of bet-hedging strategies and the cues of diapause exit to progress the representation of the SLP in global biogeochemical models.