Bottom‐up as well as top‐down processes govern zoobenthic secondary production in a tidal‐flat ecosystem

AbstractFor decades, we monitored numbers and weights of the macrozoobenthos in a vast tidal‐flat area in the Dutch Wadden Sea. We used data collected since 1992 to estimate annual secondary production. Data on chlorophyll concentrations and primary production were available for a nearby tidal inlet, showing rapid increases in the late 1970s and gradual declines since the late 1980s. The benthos responded with a doubling of biomass around 1980 and gradual declines of summer biomass, annual production as well as seasonal biomass gain for the 1992–2019 de‐eutrophication period. In nearly all years, production of bivalves (on average 25.5 gAFDM m−2 yr−1) accounted for more than half of the total zoobenthic production (averaging 42.7 gAFDM m−2 yr−1). Production values for individual years were not correlated with chlorophyll concentrations or primary production of that year, but did show a significantly positive relationship with bivalve recruitment success in the preceding year. As recruitment was governed by predation pressure on early bivalve bottom stages, it is a top‐down process. Bottom‐up processes were expressed in the gradually declining long‐term trend of bivalve growth rates and secondary production during the de‐eutrophication period and also in a few years when bivalve numbers were so high that growth rates were reduced by food competition. In by far most years, numbers of bottom animals were too low to fully exploit the carrying capacity of the system and production increased linearly with preceding recruitment.