Continental margins are disproportionally important for global primary production,
fisheries and CO2 uptake. However, across the Northeast Atlantic shelves, there has
been an ongoing summertime decline of key biota—large diatoms, dinoflagellates and
copepods—that traditionally fuel higher tropic levels such as fish, sea birds and ...
Continental margins are disproportionally important for global primary production,
fisheries and CO2 uptake. However, across the Northeast Atlantic shelves, there has
been an ongoing summertime decline of key biota—large diatoms, dinoflagellates and
copepods—that traditionally fuel higher tropic levels such as fish, sea birds and marine mammals. Here, we combine multiple time series with in situ process studies
to link these declines to summer nutrient stress and increasing proportions of picophytoplankton that can comprise up to 90% of the combined pico- and nanophytoplankton biomass in coastal areas. Among the pico-fraction, it is the cyanobacterium
Synechococcus that flourishes when iron and nitrogen resupply to surface waters are
diminished. Our field data show how traits beyond small size give Synechococcus a
competitive edge over pico- and nanoeukaryotes. Key is their ability to grow at low
irradiances near the nutricline, which is aided by their superior light-harvesting system and high affinity to iron. However, minute size and lack of essential biomolecules
(e.g. omega-3 polyunsaturated fatty acids and sterols) render Synechococcus poor
primary producers to sustain shelf sea food webs efficiently. The combination of earlier spring blooms and lower summer food quantity and quality creates an increasing period of suboptimal feeding conditions for zooplankton at a time of year when
their metabolic demand is highest. We suggest that this nutrition-related mismatch
has contributed to the widespread, ~50% decline in summer copepod abundance we
observe over the last 60 years. With Synechococcus clades being prominent from the
tropics to the Arctic and their abundances increasing worldwide, our study informs
projections of future food web dynamics in coastal and shelf areas where droughts
and stratification lead to increasing nutrient starvation of surface waters.