A green-blue link made browner: how terrestrial climate change affects marine ecology

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? By extending published data series on Northeast Arctic cod spawning phenology, we found a gradually delay in spawning time by 40 days between 1877 and 1980, after which it started advancing. We have scrutinized potential changes in the Norwegian Coastal Waters, where cod spawn, including water temperature and spring bloom conditions. These are influenced by a changing climate involving warming and precipitation, but there is also a smoking gun suggesting links to terrestrial systems. Our preliminary investigation suggests continent-wide land-sea interactions of a scale such that warmer taiga in Russia makes the Baltic Sea browner, the Norwegian Coastal Water darker, and the spring bloom in Northern Norway later, which is what made cod spawn later, and not earlier, in warm water. In a new study, published in L&O Letters (Opdal et al, 2023), we show a string relationship between forest growth in Northern Europe and water clarity in the Baltic, Kattegat and Skagerak Seas between 1900 and 2020. The more forest, the more CDOM in the freshwater systems which drain to the coastal water. Further, it seems evident that this brown freshwater and its CDOM constituents persist in the coastal waters across several thousand kilometres all the way to the Barents Sea. This connects Barents Sea to the Baltic Sea, and possibly to the Baltic lakes and forests, pointing to an interesting type of ecosystem connectivity from land to sea over great distances. It also demonstrates how forestry practices may have far-reaching downstream consequences in space and time. Another, closely related study, shows that future climate warming will cause further vegetation increase and thus an increased transport of dissolved organic material to the coastal waters - possible cuasing further coastal water darkening. In a newly submitted article currently in peer review in Global Change Biology (Opdal et al, in review) we find that the coastal water darkening of the Norwegian Coastal Waters, with links to the Baltic Sea, have observable effects on phytoplankton and fish in the maisn spawning grounds of the Northeast Arctic cod in Lofoten. There, the cod is spawning later (1877-1990), apparently in sychrony with the delayed phytoplankton spring bloom in the period 1936-1990 (no data before 1936). After 1990, we observe that the change in both the spawning and bloom timing diminishes. This is primarily due to climate warming causing an earlier stratification of the water column, which counteract the effect of coastal water darkening. Thus, we find that two anthropogenic drivers, land use change and climate warming, is butting against each other in the marine ecosystem. A closely related article to this finding, conditionally accepted in Ecology, have also shown the strong spatio-tempral synchrony between phytoplankton spring bloom and cod spawning time (Opdal et al, 2023).

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? By extending published data series on Northeast Arctic cod spawning phenology, we found a gradually delay in spawning time by 40 days between 1877 and 1980, after which it started advancing. We have scrutinized potential changes in the Norwegian Coastal Waters, where cod spawn, including water temperature and spring bloom conditions. These are influenced by a changing climate involving warming and precipitation, but there is also a smoking gun suggesting links to terrestrial systems. Our preliminary investigation suggests continent-wide land-sea interactions of a scale such that warmer taiga in Russia makes the Baltic Sea browner, the Norwegian Coastal Water darker, and the spring bloom in Northern Norway later, which is what made cod spawn later, and not earlier, in warm water. One potential cause-and-effect sequence that might link spring bloom conditions in coastal waters to terrestrial ecosystem is related to nutrient loading and the widespread, recent browning of lakes and rivers. While nutrient loading is primarily driven by agricultural practices, browning is also climate-driven via increased precipitation and air temperature, stimulating plant and forest growth. Exacerbated by land use change, this increases supply of dissolved organic matter (DOM) in lakes and rivers, which reduces water clarity (i.e. browning). Eventually, coastal water clarity is reduced directly (DOM) or indirectly (eutrophication), with consequences for plankton growth and, in turn, the timing of events such as the spring phytoplankton bloom and fish spawning. These changes have likely been reversed by warm temperatures since the 1980s, making future projections dependent on strong interactions between multiple drivers.