Posidonia oceanica meadow: a low nutrient high chlorophyll (LNHC) system?

Sylvie Gobert,Jean-Marie Bouquegneau,Noémie Laumont

doi:10.1186/1472-6785-2-9

Abstract

BackgroundIn spite of very low nutrient concentrations in its vicinity – both column and pore waters-, the Posidonia oceanica of the Revellata Bay displays high biomass and productivity. We measured the nutrient fluxes from the sediment into the water enclosed among the leaf shoots ("canopy water") to determine if it is possible source of nutrients for P. oceanica leaves.ResultsDuring the summer, the canopy water appears to act as a nutrient reservoir for the plant. During that period, the canopy water layer displays both a temperature 0.5°C cooler than the upper water column, and a much higher nutrient content, as shown in this work using a very simple original technique permitting to sample water with a minimal disturbance of the water column's vertical structure.Despite low nutrient concentrations in pore water, mean net fluxes were measured from the sediment to the canopy water. These fluxes are sufficient to provide 20% of the mean daily nitrogen and phosphorus requirement of the P. oceanica shoots.ConclusionAn internal cycling of nutrients from P. oceanica senescent leaves was previously noted as an efficient strategy to help face low nutrient availability. The present study points out a second strategy which consists in holding back, in the canopy, the nutrients released at the water-sediment interface. This process occurs when long leaves, during poor nutrient periods in the water column, providing, to P. oceanica, the possibility to develop, high biomass, high chlorophyll quantities in low nutrient environment (a Low Nutrients High Chlorophyll system).

Highlights

In spite of very low nutrient concentrations in its vicinity – both column and pore waters, the Posidonia oceanica of the Revellata Bay displays high biomass and productivity
Compared to other meadows [4], the nutrient pore water concentrations are very low in the Revellata Bay while its canopy displays a biomass as large as many other meadows in the
We have examined the nutrient conditions in the vicinity of the P. oceanica meadow of the Revellata Bay and we have estimated the nutrient fluxes from the sediment into the canopy using a benthic chamber

Summary

Introduction

In spite of very low nutrient concentrations in its vicinity – both column and pore waters-, the Posidonia oceanica of the Revellata Bay displays high biomass and productivity. A nitrogen and silica limitation of the surface waters, occurring in the last decade, has been pointed out with a drastic reduction of phytoplankton biomass in relation with an increase of the sea surface water temperature. The nutrients and the chlorophyll a concentrations in the water column were low, even during the winter-spring phytoplankton bloom (i.e. 0.2–0.3 μmol L-1 for nitrate and

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Conclusion