Continuity in the photosynthetic production of dissolved organic carbon from eutrophic to oligotrophic waters

Abstract

, PER was relatively constant at ~20%, and integrated DOCp could be predicted from integrated POCp with the equation: log (DOCp) = 0.96 × log (POCp) -0.51 (r 2 = 0.90, n = 35, p < 0.001). KEY WORDS: Phytoplankton · Dissolved organic carbon · Oligotrophic conditions · Celtic SeaResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 299: 7-17, 2005 zone does not allow one to conclude unequivocally which environmental factors control the vertical variability in the relative importance of dissolved primary production. Additional approaches are therefore required, such as the experimental determination of the relationship between different environmental factors (e.g. temperature, irradiance, nutrient concentration) and DOCp (Zlotnik & Dubinsky 1989, Obernosterer & Herndl 1995, the monitoring of DOCp over light -dark cycles (Mague et al. 1980, Marañón et al. 2004, and the analysis of the biochemical composition of the released compounds (Obernosterer & Herndl 1995, Meon & Kirchman 2001. Using some of these approaches, in conjunction with the determination of the vertical and seasonal variability of DOCp, we have recently addressed the significance and mechanisms of DOCp in a coastal, eutrophic ecosystem (Marañón et al. 2004). We observed that a high percentage (> 80%) of the variability in DOCp could be explained by the changes in the rate of particulate primary production (POCp). We also found that the relative contribution of DOCp to total, integrated primary production (PER) was relatively constant at around 20%, irrespective of phytoplankton biomass, size-structure and productivity. Moreover, there was no DOC accumulation during the dark period, indicating that the release of recent photosynthate was a purely physiological, light-dependent process and that trophic mechanisms were not involved in DOCp. Finally, DOCp took place throughout the euphotic layer and across a wide range of irradiances. This suggested that photosynthate release by phytoplankton occurs as a background process of passive diffusion or leakage (Raven 1986), rather than representing an overflow mechanism when C fixation is high and nutrient supply is low (Wood & Van Valen 1990).In order to determine if the mechanisms and quantitative importance of phytoplankton DOCp observed in a coastal, eutrophic ecosystem are also applicable to a wider range of environments, it is critical to investigate DOCp under oligotrophic conditions. Low primary production rates (≤ 0.5 g C m -2 d -1) are common throughout most of the year over large expanses of the open ocean and are also characteristic of coastal and shelf seas during summer. The importance of oligotrophic regions (defined as those whose mean daily C fixation rate is ≤ 0.5 g C m -2 d -1) is illustrated by the fact that they account for ~50% of global marine primary production (see Longhurst et al. 1995, their Table 1). It is therefore of interest to determine if DOCp in low-productivity waters occurs at a similar relative rate and through the same mechanism...

Heterotrophic bacterial production in the eastern South Pacific: longitudinal trends and coupling with primary production

“…Estimates on the percent of primary production released as DOC vary largely among studies. Although some studies report a percentage of excretion constant across trophic gradients (Maranon et al, 2005); both laboratory (Myklestadt, 1995;Obernosterer and Herndl, 1995) and field studies (Teira et al, 2001;Moran et al, 2002;Fernández et al, 2004), indicate an increase of the percent of primary production released as DOC in nutrient-limited environments. In the South Pacific Gyre primary production was strongly limited by nitrogen (Bonnet et al, 2007).…”

Dissolved organic matter (DOM) release by phytoplankton in the contemporary and future ocean

“…For example, Baines & Pace (1991) concluded that DOM release is constrained by the total availability of photosynthates, rather than phytoplankton biomass, and therefore active processes are more important than diffusion. However, constant PER throughout the water column, which only increased at very low light conditions, led to the conclusion that DOC release was the result of passive diffusion rather than photosynthetic overflow (Marañón et al, 2004(Marañón et al, , 2005. The passive diffusion model implies that a population dominated by small cells should have a higher PER than populations of larger cells.…”

“…Total primary production (TPP) can be divided into particulate primary production (PPP) and dissolved primary production (DPP). PER is simply DPP/TPP × 100 (Marañón et al, 2005;Morán et al, 2006). While PER is a useful parameter to understand the physiology of phytoplankton, it is not as useful as the absolute measures of carbon flux for understanding the biogeochemistry of ecosystems.…”

A dual‐labeling method for the simultaneous measurement of dissolved inorganic carbon and phosphate uptake by marine planktonic species

“…Because the methodological problems associated with 24-h incubation experiments affect both carbon (Moutin et al 1999, Maranon et al 2005 and phosphate (Nalewajko andGarside 1983, Harrison andHarris 1986) uptake measurements, we decided to measure hourly rates, at noon. Losses remain negligible for short-term incubations (Nalewajko and Garside 1983, Harrison and Harris 1986, Moutin et al 1999, Maranon et al 2005 and consequently, these rates are close to gross rates.…”