The spring bloom in the Antarctic Polar Frontal Zone as observed from a mesoscale array of bio-optical sensors

Abstract

The US Joint Global Ocean Flux Study (JGOFS) conducted a series of survey and process studies in part to understand the processes regulating primary productivity and carbon flux in the APFZ, which is a high-nutrient, low-chlorophyll (HNLC) region. We deployed a high-resolution array of 12 moorings (average horizontal spacing 30 km) equipped with bio-optical and physical sensors to study the temporal and spatial scales of biological and physical processes in the APFZ. The moorings collected data from November 1997 to March 1998, effectively observing the growing season. Estimates of chlorophyll and sun-stimulated fluorescence/chlorophyll (F/C) were derived from the bio-optical measurements. Each mooring showed a strong spring bloom beginning in early December as the upper ocean began to stratify, with chlorophyll levels nearly quadrupling. The time series, along with ship studies, suggest that phytoplankton were initially light-limited as a result of deep, late spring mixing, followed by intense zooplankton grazing or silicate limitation, which controlled the maximum chlorophyll concentration, and finally by iron limitation, which led to increasing photoadaptive stress. These results suggest that phytoplankton in the APFZ are regulated by a confluence of processes involving light, grazing, silicate, and iron, and that models comprising a single mechanism may not be sufficient. The spring bloom in the APFZ is a transient event, persisting for only a few weeks, and therefore it is difficult to draw conclusions from sporadic ship cruises. Moreover, its spatial scales are also small so that widely spaced hydrographic stations can easily overlook critical processes.