A box-model of carrying capacity of the Thau lagoon in the context of ecological status regulations and sustainable shellfish cultures

• Published in: Ecological modelling 2020-06, Vol.426, p.109049, Article 109049
• Main Authors: Pete, Romain, Guyondet, Thomas, Bec, Beatrice, Derolez, Valérie, Cesmat, Ludovic, Lagarde, Franck, Pouvreau, Stéphane, Fiandrino, Annie, Richard, Marion
• Format: Article
• Language: English
• Subjects: Biodiversity and Ecology; Carrying capacity; Coupled biogeochemical-deb model; Ecological indicators; Environmental Sciences; Nitrogen and phosphorus inputs; Phytoplankton depletion; Shellfish farming; Stakeholder implication; Water frame directive
• ISSN: 0304-3800; 1872-7026
• DOI: 10.1016/j.ecolmodel.2020.109049

•The variability of oyster production is mainly controlled by the hydrometeorology.•Waste water treatment plant (WWTP) efficiency affect oyster production, without significantly improving the ecological status of the Thau lagoon.•Scenario of oyster stocking density to its authorized maximum triggered an increase of oyster productions at the expenses of thinner oysters with a lower condition index.•The Thau lagoon remains an exceptional site for oyster farming associated with “good” ecological status condition and where the carrying capacity was not limiting.•Oyster production might be threatened by drastic measures to be undertaken on WWPT, particularly on phosphorus that seems to control primary production in the Thau ecosystem. The decrease of microbial and nutrient inputs from the watershed has long dominated lagoon ecosystem management objectives. Phytoplankton biomass and abundance have drastically decreased for more than a decade and Zostera meadow have gradually recovered, expressing lagoon ecosystem restoration such as Thau lagoon. Do the progressive achievement of the good ecological status of the Thau lagoon possibly threatens the shellfish industry in terms of production and oyster quality, by reducing the carrying capacity? To provide answers about the right balance to be achieved between conservation and exploitation, a new numerical tool was developed to help in decision-making. We hereby propose to incorporate a Dynamic Energy Budget type shellfish production model to an existing lagoon ecosystem box-model. The influence of different scenarios of nutrient inputs (related to projections of population growth or improvement of treatment plants) and shellfish stocks were tested on oyster performances (production, oyster condition index), carrying capacity of the lagoon and ecological status indices used within the EU Water Framework Directive. Model outputs demonstrated that shellfish production was mainly controlled by nutrient inputs, which depend on hydro-meteorological variability, and specifically by phosphorus and N:P ratios of nutrient inputs. Scenarios tested, however, demonstrated smaller differences of oyster production in comparison to inter-annual variability. The overall ecological status of the lagoon remained in a “good” status with acceptable lagoon-scale phytoplankton depletion, regardless of scenarios, setting the carrying capacity of this ecosystem to be sustainable.