Could long term shift in cod phenology be linked to terrestrial greening and coastal browning

Summary

With climate warming, a widespread expectation is that events in spring, like flowering, bird migrations, and insect bursts, will occur earlier because temperature permits. What then, when data shows the opposite? Here we show a steady delay in spawning time of Northeast Arctic cod by more than 40 days between 1877 and 1980 – a period in which there is no regional trend in ocean temperatures. Our preliminary investigation point towards a potential link between increased terrestrial vegetation, via coastal water darkening to phytoplankton spring bloom and cod spawning time. Based on centennial observations of Secchi disk depth and phytoplankton concentrations, we find that the long-term reduction in coastal water clarity in these areas are primarily driven by other factors than phytoplankton, including dissolved organic matter of terrestrial origin. From a state-of-the-art water column model, we estimate that the reduction in water clarity alone would result in a 3 weeks delay in the phytoplankton spring bloom onset over the 20th century – in line with the delay in cod spawning time and opposite predictions from global warming. We hypothesize that the widespread, recent browning of lakes and rivers, primarily driven by land use change and increased precipitation and air temperature stimulating plant and forest growth, eventually drain to the coast, reducing coastal water clarity, affecting plankton growth conditions with implications for phenological events such as timing of the spring phytoplankton bloom and ultimately fish spawning time.