Potential impacts of the spring-neap tidal cycle on shelf sea primary production.

Spring-neap modulation of tidal mixing could potentially have significant effects on the timing and magnitude of primary production in stratified shelf seas. A 1-dimensional turbulence model, coupled to a simple model of primary production, is used to identify potential spring-neap impacts. Changes in the timing of the spring-neap cycle could contribute 10% of inter-annual variability of bloom timing in weak tidal regimes and 25% in areas with stronger tidal currents. In stratified regions away from the tidal mixing fronts the spring-neap cycle is predicted to result in periodicity in the biological rates within the thermocline, and the turbulent flux of organic carbon into the bottom water. The strongest impacts are predicted within 15 – 50 km of the tidal mixing fronts, with increases in sub-surface primary production and carbon export. At the fronts there is substantial extra primary production driven by the spring-neap cycle, contributing an extra 70% annually compared to fronts forced by the M2 tide only. This impact is reflected in the organic carbon mixed downward into the bottom waters near the front. The results have important implications for the interpretation of observations of primary production, and for the resolution required by shelf-wide models of the marine ecosystem.