Response of diatom and dinoflagellate lifeforms to reduced phosphorus loading: a case study in the Thau lagoon, France

Type Article
Date 2015-09
Language English
Author(s) Gowen R. J.1, Collos Y.2, Tett P.3, Scherer C.1, Bec B.2, Abadie EricORCID4, Allen M.5, O'Brien T.6
Affiliation(s) 1 : Agrifood & Biosci Inst, Fisheries & Aquat Ecosyst Branch, SAFSD, Belfast BT9 5PX, Antrim, North Ireland.
2 : Univ Montpellier, IFREMER, CNRS, IRD,UMR 9190,MARBEC, F-34095 Montpellier 5, France.
3 : Scottish Assoc Marine Sci, Scottish Marine Inst, Oban PA37 1QA, Argyll, Scotland.
4 : IFREMER, Lab Environm Ressources, F-34203 Sete, France.
5 : Agrifood & Biosci Inst, Biometr Informat & Syst Branch, FCAD, Belfast BT9 5PX, Antrim, North Ireland.
6 : NOAA, Natl Marine Fisheries Serv, Silver Spring, MD USA.
Source Estuarine Coastal And Shelf Science (0272-7714) (Academic Press Ltd- Elsevier Science Ltd), 2015-09 , Vol. 162 , N. 5 , P. 45-52
DOI 10.1016/j.ecss.2015.03.033
WOS© Times Cited 16
Note Special Issue: Global Patterns of Phytoplankton Dynamics in Coastal Ecosystems
Keyword(s) eutrophication, Thau lagoon, phosphorus, lifeform, state-space, Euclidean distance
Abstract The basin of Thau in southern France is a shallow, weakly flushed lagoon which is an important location for oyster cultivation. Phytoplankton analyses were carried out in 1975-1976 and then (almost) continuously since 1987. We report an investigation of 'the balance of organisms' in phytoplankton in relation to reductions in phosphorus loading, using two new tools based on phytoplankton lifeforms: the Plankton Index for Phytoplankton (PIp); Euclidean distance in state space. Our results show the utility of the tools for analysing changes in the ‘balance of organisms’ at the level of functional groups (in our study diatoms and dinoflagellates), but also illustrate the difficulties in demonstrating the reversal of human impacts resulting from eutrophication.

The comparison between 1987-89 and 1976 showed the expected 'de-eutrophication' due to the reduction in dissolved inorganic phosphate (DIP), with a decrease in dinoflagellate abundance. Since 1989, year-to-year variation in annual mean concentrations of (DIP) may have contributed to inter-annual variability in the balance of the two lifeforms, but the data suggest that the system has remained in a dynamically stable regime because: (i) there was no long-term trend in Euclidean distance from the reference; and (ii) there was no increase in inter-annual variability about the time-series mean Euclidean distance suggesting there has been no decrease in resilience which might signal a regime change.

Integrated management of human activities will be required to manage (and reduce) total P in the system. Monitoring phytoplankton and nutrients concentrations to determine how primary production and the balance of species respond to further changes in the nutrient status of the lagoon should be an integral part of any management programme.
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