Phytoplankton Productivity and Photophysiology in the Surf Zone of Sandy Beaches in North Carolina, USA

Abstract

Measurements of primary production in surf-zone habitats are relatively rare and often utilize simulation approaches, owing to the physical challenges of working in surf. The study reported here examined primary production in situ at two open ocean sandy beaches in southeastern North Carolina during relatively calm summer conditions. In situ bottle incubations using 14C uptake methods were complemented by simultaneous measures of phytoplankton photo-physiology assessed by Fast Repetition Rate Fluorometry (FRRF) in flow-through mode at the two sites across a spring-neap tidal cycle in July, 2010. The surf-zone phytoplankton was dominated by small centric and pennate diatoms as well as cyanobacteria and chlorophytes with biomass concentrations of 3.63–9.23 mg chl a m−3. Primary productivity was relatively high, ranging from 31.5–88.0 mg C m−3 h−1 by 14C. Biomass-specific productivity averaged ∼9.4 mg C (mg chl a)−1 h−1 by 14C, indicating healthy phytoplankton populations. Measurements of the functional absorption cross section of photosystem II, σPSII, via FRRF were 327–380, comparable to values reported by other investigators of open ocean phytoplankton. Averaged values of the maximum effective quantum yield, Fv/Fm, corresponded to proportions of photochemically competent PSII reaction centers of 62.6 % to 72 %, indicating that the phytoplankton were nutrient-replete. These data suggest that the surf zone, although a spatially confined habitat, is a productive one that plays a significant role in coastal ocean ecology. Further investigation is needed to better understand primary productivity of phytoplankton in the surf zone and the effect of the dynamic environment on their physiological responses.