Adaptive model of DOM dynamics in the surface ocean

The bulk of marine dissolved organic matter (DOM) forms a large, old, refractory carbon pool in the deep ocean, yet a small fraction in the surface ocean is actively involved in the global carbon cycle and may contribute significantly to the biological pump. We argue that present models of plankton and DOM in the surface ocean are incompatible with current knowledge of marine DOM dynamics. We present a plankton model with an adaptive formulation of bacteria-DOM interactions which is more consistent with observations. Our model reproduces net accumulation of DOM and is the first to reconcile the prevailing reports of net consumption of inorganic nitrogen by bacteria with commonly found DOC:DON ratios in the surface ocean. Our model predicts that factors governing DOM production by phytoplankton and zooplankton have little influence on DOM accumulation in the surface ocean. Long-term accumulation, eventual export of DOM, and hence its contribution to the biological pump appear to be primarily controlled by characteristics of bacterial DOM utilization. The model implies a negative relation between temperature and DOM accumulation, which can be obscured, however, by the impact of temperature on water-column stability. On longer time scales, this negative relation could indicate a positive feedback between temperature and CO2. DOM can accumulate independently of inorganic nutrient content of the surface ocean. Therefore, the predicted positive feedback is potentially very strong and could help explain the large variations in atmospheric CO between glacial and interglacial periods

Effect of a freshwater pulse on mesoscale circulation and phytoplankton distribution in the lower St. Lawrence Estuary

As part of a multidisciplinary program to study the physical-biological interactions regulating carbon flows in the lower St. Lawrence Estuary (LSLE), three cruises were conducted in June–July 1990 during a neap-spring tidal cycle when biological production was expected to be maximal. Nutrient (nitrates and silicates), phytoplankton biomass (chlorophyll), oxygen, temperature, salinity, and current fields were used to elucidate the effect of a freshwater pulse produced by the discharge of the St. Lawrence and Saguenay rivers on the current fields and the biological variability and productivity of the LSLE. A simple Rossby adjustment model is presented to explain the temporal (3–5 days) and spatial (40–50 km) scales of motion in our study region (impact of the freshwater pulse on the circulation). Prior to the passage of the pulse during the neap tide, the circulation was dominated by a downstream outflow and phytoplankton blooms were limited to areas of weak baroclinic currents downstream and along the south shore. The arrival of the pulse during the tidal transition led to the intensification of a transverse current that most likely reduced flushing and allowed phytoplankton biomass to develop further upstream and toward the north shore. During the spring tide, lower salinity waters and the bloom spread along the north shore as the transverse current weakened. Based on these observations, a new conceptual model of mesoscale physical-biological interactions in the LSLE is presented that emphasizes the importance of transverse motions in regulating mesoscale patterns in phytoplankton blooms

The mean function provides robustness to linear inverse modelling flow estimation in food webs: A comparison of functions derived from statistics and ecological theories

Quantitative estimates of carbon flows within food webs are increasingly viewed as essential to progress on a number of questions in basic and applied ecosystem science. Inverse modelling has been used for more than 20 years to estimate flow values for incomplete data sets. Monte Carlo Markov Chain linear inverse modelling calculates a probability density function for each flow. Among this distribution of possible values for each flow, the mean is generally chosen when a single solution is needed. The objective of the present study is to compare the robustness of the result when using the mean function, compared with 2 other statistical functions and 7 ecological functions derived from ecological theories on ecosystem maturity. The performance of the various functions was tested by comparing their accuracy in reconstructing a complete data set, the marine food web of Sylt-Rømø Bight, with known flows systematically removed. This was carried out on seven habitats and for 4 levels of degradation of the information. The robustness of each function was measured by comparing the estimated values of flows from inverse modelling after degradation with values from the original, complete data set. The analysis of results shows that the error of the estimated flows increases with the degradation of information, independent of the considered function. Two functions, the mean and the system omnivory index, provide more precise results than the others independent of the level of degradation of the information considered. The mean had the least impact on the reconstruction of food web flow values and on their organization described by ecological network analysis indices. © 2013 Elsevier B.V