Large variability in continental shelf production of phytoplankton carbon revealed by satellite

Abstract. We estimate the net production of phytoplankton in the Gulf of Maine (GoM) over a 3-yr period using satellite ocean color data in conjunction with surface velocities from a high-resolution operational ocean circulation model. Chlorophyll (Chl-a) and light attenuation (K₄₉₀) products are combined with a carbon to chlorophyll model to estimate the phytoplankton carbon (PC) stock in the euphotic layer. A satellite-based productivity, termed NCP_e in analogy with net community production (NCP), is derived by tracking changes in satellite-derived PC from one satellite image to the next, along water parcel trajectories calculated with surface velocities from the ocean circulation model. Such an along-trajectory analysis of satellite data discounts the effect of advection that would otherwise contribute to the temporal change between consecutive images viewed in the fixed reference frame. Our results show a high variability of up to ±500 mg C m⁻² d⁻¹ in NCP_e on spatial scales of 10–100 km. A region-wide median NCP_e of 40–50 mg C m⁻² d⁻¹ is often prevalent in the Gulf, while blooms attain peak values of 400 mg C m⁻² d⁻¹ for a few days. The spatio-temporal variability of NCP_e in this region, though conditioned by seasonality, is dominated by events lasting a few days, which if integrated, lead to large inter-annual variability in the annual carbon budget. This study is a step toward achieving synoptic and time-dependent estimates of oceanic productivity and NCP from satellite data.