Premium
Significance and mechanisms of photosynthetic production of dissolved organic carbonin a coastal eutrophic ecosystem
Author(s) -
Marañón Emilio,
Cermeño Pedro,
Fernández Emilio,
Rodríguez Jaime,
Zabala Laura
Publication year - 2004
Publication title -
limnology and oceanography
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.7
H-Index - 197
eISSN - 1939-5590
pISSN - 0024-3590
DOI - 10.4319/lo.2004.49.5.1652
Subject(s) - dissolved organic carbon , phytoplankton , photic zone , irradiance , eutrophication , upwelling , productivity , bacterioplankton , trophic level , environmental science , biomass (ecology) , photosynthesis , environmental chemistry , ecosystem , plankton , nutrient , oceanography , chemistry , ecology , biology , botany , geology , physics , macroeconomics , quantum mechanics , economics
We have determined the seasonal (July 2001‐July 2002) and vertical variability in the photosynthetic production of dissolved organic carbon (DOC p ) and particulate organic carbon (POC p ) in a coastal upwelling ecosystem (Ría de Vigo, Northwest Spain), together with the relationship between irradiance and DOC p and the time‐course of DOC p over 24‐h periods. Euphotic layer‐integrated rates of DOC p and POC p ranged between 5 and 190 mg C m −2 h −1 and between 40 and 1,130 mg C m −2 h −1 , respectively. Irradiance was the most important variable affecting the vertical variability of the percentage of extracellular release [PER, defined as DOC p /(DOC p + POC p )]. Whereas POC p decreased markedly below the surface, DOC p remained constant or even increased, thus causing a sharp increase in PER with depth. Biomass‐specific rates of DOC production also increased with depth. These observations were confirmed by the results of photosynthesis‐irradiance experiments, which consistently showed highest DOC p and PER values at subsaturating irradiances. Our results argue against the view that the release of DOC is an overflow mechanism occurring preferentially under conditions of high irradiance and low nutrient concentration. PER was uncorrelated with the size structure of phytoplankton biomass and productivity, and >80% of the variability in integrated DOC p was explained by POC p . These findings indicate that the relative importance of dissolved primary production was independent of the dominant type of planktonic trophic organization. Moreover, production of DOC stopped at night, which strongly indicates that trophic processes were not involved in the release of dissolved photosynthate. Our data support a purely physiological mechanism of passive DOC release by normally growing cells, which is enhanced under suboptimal irradiances but proceeds at a similar biomass‐specific rate throughout the year. On an integrated basis, PER averaged 19 ± 1%, thus indicating that even in eutrophic waters, total primary productivity can be significantly underestimated if the dissolved products of photosynthesis are not taken into account.