DMSP synthesis and exudation in phytoplankton:a modeling approach

In the marine environment, phytoplankton are the fundamental producers of dimethylsulfoniopropionate (DMSP), the precursor of the climatically active gas dimethylsulfide (DMS). DMSP is released by exudation, cell autolysis, and zooplankton grazing during phytoplankton blooms. In this study, we developed a model of phytoplankton DMSP and DMS production allowing quantification of the exudation rates of these compounds during different growth phases. The model was tested on published data from axenic cultures of Prorocentrum minimum and Phaeocystis sp.; DMSP exudation rates vary considerably between the 2 species. Model results show that P. minimum exudes around 1% d(-1) of its DMSP quota during the latent, exponential and senescent phases. This is comparable to the average exudation rate estimated from previous laboratory experiments. However, Phaeocystis sp. exudes from 3 to 11% d(-1) of its DMSP quota. For this species, DMSP exudation rates apparently show an inverse relationship with the population growth rate. The maximum DMSP exudation rate in Phaeocystis sp. is 10 times higher than previously reported DMSP or DMS exudation rates. Our results suggest that exudation may be as important as cell autolysis in the release of DMSP during Phaeocystis sp. blooms. We conclude that exudation should be incorporated in models of DMS cycling in the marine environment. Moreover, our results for Phaeocystis sp. suggest that a low and constant exudation rate, as sometimes used in models, is not suitable for all conditions

Quantifying Food Web Flows Using Linear Inverse Models

The quantitative mapping of food web flows based on empirical data is a crucial yet difficult task in ecology. The difficulty arises from the under-sampling of food webs, because most data sets are incomplete and uncertain. In this article, we review methods to quantify food web flows based on empirical data using linear inverse models (LIM). The food web in a LIM is described as a linear function of its flows, which are estimated from empirical data by inverse modeling. The under-sampling of food webs implies that infinitely many different solutions exist that are consistent with a given data set. The existing approaches to food web LIM select a single solution from this infinite set by invoking additional assumptions: either a specific selection criterion that has no solid ecological basis is used or the data set is artificially upgraded by assigning fixed values to, for example, physiological parameters. Here, we advance a likelihood approach (LA) that follows a different solution philosophy. Rather than singling out one particular solution, the LA generates a large set of possible solutions from which the marginal probability density function (mPDF) of each flow and correlations between flows can be derived. The LA is exemplified with an example model of a soil food web and is made available in the open-source R-software. Moreover, we show how stoichiometric data, stable isotope signatures, and fatty acid compositions can be included in the LIM to alleviate the under-sampling problem. Overall, LIM prove to be a powerful tool in food web research, which can bridge the gap between empirical data and the analysis of food web structures.

Inverse Modeling in Modern Ecology and Application to Coastal Ecosystems

International audienceQuantitative estimates of energy or material flows within food webs are increasingly viewed as essential to progress on a number of questions in ecosystem science. Inverse analysis has been used since the 1980s to estimate all flows within plankton food webs originally based on incomplete information. Its application to many aquatic environments, including the coastal zone, has led to a variety of methodological improvements. This chapter explains the methodology of inverse modeling and illustrates its application in modern ecosystems ecology. This relatively new approach also provides rigorous statistical comparisons of food web properties across ecosystems. © 2011 Elsevier Inc. All rights reserved

Importance of mesozooplanckton feeding for the downward flux of biogenetic carbon in the Gulf of St. Lawrence (Canada)

We tested the importance of mesozooplankton feeding and defecation for the downward flux of biogenic carbon (C) in the Gulf of St. Lawrence, a marine coastal environment characterized by high zooplankton abundance. Five stations were sampled over nine cruises between July 1992 and June 1994. The rates of chlorophyll (Chl) ingestion and C defecation were determined for the major copepod species. Free-drifting, short-term (24　h) sediment traps were deployed at the sampled sites at 50 and 150　m, and the trap contents were analyzed for fecal pellets (microscopy), Chl and pheopigments (chromatography). The flux at 50　m of C associated with fecal pellets ranged between 5 and 138　mg C m-2 d-1, while total POC flux varied between 78 and 302　mg C m-2 d-1. Of the total POC found in sediment traps at 50　m, close to 50%, on average, was composed of zooplankton fecal pellets. In contrast, direct algal (Chl) sinking was <10%. The fecal pellet contribution varied seasonally, with a maximum (>73%) in June 1994 at all stations. This maximum did not necessarily coincide with the highest rate of production of fecal pellets in the surface waters, indicating that other factors influenced the sinking of fecal pellets. A shallow mixed-layer depth seemed to favor fast removal from the surface. The average fecal pellet contribution to the total downward POC flux was slightly higher for stations 4 and 5 (52% compared with 39% for the other stations). These stations were characterized by zooplankton communities that differed from those of the other stations, smaller copepods such as Temora longicornis frequently dominated at stations 4 and 5, Calanus spp. dominated at stations 1, 2 and 6. High fluxes of fecal pellets frequently occurred when the index of herbivory (Chl ingestion　:　total C ingestion) was low, transferring downwards carbon of heterotrophic origin. The contribution of C-transformed pheopigments to total POC flux was low (6% on average). It co-varied with the degree of herbivory in the overlying waters. The production of fecal pellets in the top 50　m co-varied with Chl ingestion, but not with the index of herbivory, consistent with an algal food intake representing <50% of total C ingested. Phytoplankton ingestion was highest when primary production (not algal biomass) was maximum. In consequence, mesozooplankton feeding did not exert a strong control over the phytoplankton biomass of the Gulf of St. Lawrence (at most 10%), but it was more closely tied to primary production. Phytoplankton biomass is more likely controlled by physical factors in this region. Mesozooplankton fecal pellets contributed significantly to the downward flux of biogenic C all through the year in this environment, transferring alternatively C of autotrophic and heterotrophic origin