In situ estimates of kelp forest productivity in macro-tidal environments

Accurately quantifying primary productivity of marine macrophytes is essential to further understanding of coastal ecosystem functioning and will allow its inclusion within marine ecosystem models. Traditional quantification of carbon standing stock and biomass accumulation only allow rough estimates of net primary production whereas incubations that measure photosynthesis in the laboratory lack realism and underestimate true photosynthetic rates. We developed in situ photorespirometry methods to estimate productivity of Laminaria hyperborea, the dominant forest-building kelp species in Northeast Atlantic. We show that sub-canopy individuals are better adapted to low light levels with higher photosynthetic efficiency and lower compensation point than larger canopy individuals. Laboratory incubations of L. hyperborea stipes revealed that productivity rates could be positive or negative, even at saturating irradiances, depending on whether they were dominated by autotrophic or heterotrophic communities, respectively. Scaling up productivity estimates based on local population demographics resulted in mean daily net productivity rates of 13.3 and -30.1 g C m(-2) d(-1), for shallow L. hyperborea forest during late summer and autumn, respectively. At saturating irradiances in the late summer-autumn study period, estimates of net and gross productivity reached 9.4 and 12 g C m(-2) h(-1), respectively. Sub-canopy individuals were responsible for 12% of estimated daily productivity during summer, but 20% in autumn. We conclude that in situ photorespirometry provides a more direct measurement of production, unaffected by exudation of dissolved carbon or tissue loss, that can account for sub-canopy and stipe components of the kelp forest, commonly excluded from traditional studies.