Implications of the depth of the euphotic zone and compensation irradiance on primary productivity in Cuenca Alfonso, BCS, Mexico.

Abstract

The euphotic zone of marine ecosystems has been defined as the layer between the water surface and the depth to which 1% of solar irradiance penetrates. However, it has been proposed that this should be extended to 0.1% to include phytoplanktonic communities adapted to low irradiances that contribute significantly to primary productivity. Sampling was conducted during April, May, September, and November, 2016, in Cuenca Alfonso at depths of 100, 33, 3, and 0.1% of active photosynthetic radiation, and at the standard depth of 150 m. CTD casts were performed and samples were collected to estimate chlorophyll-a concentrations and primary productivity. A mixed layer was observed at less than 16 m in April, May, and September that increased to 48 m in November. The highest chlorophyll-a concentrations were recorded at 33 and 3% and in one case at the 0.1% level. The microphytoplanktonic fraction concentration of chlorophyll had values that ranged from undetectable to 0.084 mg m^-3, while the picophytoplanktonic fraction measured between 0.012 and 0.33 mg m^-3. Microphytoplanktonic primary productivity varied from 0.25 to 0.41 mg C m^-3 h^-1 (scored values) and 22.39-41.69 mg C m^-2 h^-1 (integrated values). The picophytoplanktonic fraction varied from 0.25-0.34 mg C m^-3 h^-1 to 19.92-34.44 mg C m^-2 h^-1. The concentration of chlorophyll-a was significantly greater in the picophytoplanktonic fraction than in the microphytoplanktonic fraction (p<0.05); but was not significant in primary productivity (p=0.05). We conclude that the depth of the euphotic zone directly influences the productive capacity of the ecosystem, and propose deepening the zone’s profundity to at least 0.1% of surface irradiance since this could mean an increase of as much as 30% in total productivity.