| Summary: | At high latitudes, the suitable window for timing reproductive events is particularly narrow, promoting tight synchrony between trophic levels. Climate change may disrupt this synchrony due to diverging responses to temperature between e.g. the early life stages of higher trophic levels and their food resources. Evidence for this is equivocal, and the role of compensatory mechanisms are poorly understood. Here, we show how a combination of ocean warming and coastal water darkening drive long-term changes in phytoplankton spring bloom timing in Lofoten Norway, and how spawning time of Northeast Arctic cod responds in synchrony. Spring bloom timing was derived from hydrographical observations dating back to 1936, while cod spawning time was estimated from weekly fisheries catch and roe landing data since 1877. Our results suggest that land use change causing coastal water darkening has gradually delayed the spring bloom up to 1990 after which ocean warming has caused it to advance. The cod appear to track phytoplankton dynamics by timing gonadal development and spawning to maximize overlap between offspring hatch date and predicted resource availability. This finding emphasises the importance of land-ocean coupling for coastal ecosystem functioning, and the potential for fish to adapt through phenotypic plasticity. Funding provided by: The Research Council of Norway Crossref Funder Registry ID: https://ror.org/00epmv149 Award Number: 287490 Andenes surface temperature The monthly mean sea surface temperature (C°) from the Andenes lighthouse (69.3 °N, 16.1 °E) for the years 1868 to 1963 where drawn from two sources. For the period 1868-1945, temperatures where drawn directly from the publication Means and extremes of sea temperature by the Norwegian coast tables 291a and 291b (pages 61-62) by E. Frogner (1948, Geofysiske publikasjoner Vol. XV Issue 3 Pages 5-74). The raw data consisted of 8-hourly to daily observations, but are processed to monthly means by E: Frogner, without uncertainty. The wherabouts of the ...
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