Structure-dependent phytoplankton photosynthesis and production rates: implications for the formation, maintenance, and decline of plankton patches

Photosynthesis-irradiance relationships (P vs. E) were quantified for taxonomically diverse phytoplankton communities to determine whether differential net growth could be a mechanism leading to phytoplankton patchiness. The average P vs. E parameters from 3 field seasons in summer 2007 and 2008 and spring 2009 in a shallow fjord (East Sound, Washington, USA) suggested a high light-adapted but light-limited phytoplankton community with maximum photosynthesis rate (P B max ) of 3.80 ( 0.19 SE) mg C (mg chl a) -1 h -1 ; light utilization coefficient () of 0.013 ( 0.0008) mg C (mg chl a) -1 h -1 (mol photons m -2 s -1 ) -1 , and light saturation index (E k ) of 391 ( 24) mol photons m -2 s -1 . P B max and for phytoplankton communities inside patches were 30% higher than for those outside patches. Primary production was higher inside patches due to both an intrinsically higher photosynthetic capacity and exposure to higher light intensities. Patches only occupied ~12% of the water column but generated > 50% of total water column production (1117 mg C m -2 d -1 ). In the absence of advective or predatory losses, primary production within patches was sufficient to support formation and persistence of discrete patches. However, additional processes such as vertical shear are required to enforce continued separation of water masses. Patches contributed the majority of depth-integrated water column primary production, making the study of patchiness an important topic for understanding and quantifying phytoplankton abundance, distribution, and production as well as food web and biogeochemical ramifications. These results extend prior findings documenting enhanced heterotrophic grazing in patches and suggest that phytoplankton patches are not only consumption but also production hot spots.