Testing the saprobity hypothesis in a Mediterranean lagoon: effects of confinement and organic enrichment on benthic communities.
The macrobenthic community was compared at four sites characterized by varying degrees of freshwater input, organic enrichment and confinement in the Cabras lagoon (Sardinia, Italy). Three sites, riverine (C1), confined (C2) and seaward (C3), were studied on two dates of summer 2010 and on two dates of winter 2011. A fourth site (C12), representative of the central sector of the Cabras lagoon, was included in this study using the extensive historical datasets at our disposal from previously published work. We aimed to test the hypothesis that (1) the benthos is distributed according to the recently proposed concept of habitat saprobity for coastal lagoons that unifies the Pearson-Rosenberg (sensu organic enrichment) and Guélorget-Perthuisot (sensu confinement) models, and (2) indicator species of different saprobic levels can be identified among dominant species occurring along the saprobity gradient. Salinity was also considered as an additional agent of selection in brackish environments. Irrespective of significant seasonal changes within each site, our results highlighted major environmental and biotic differences between sites. At the northward riverine site (C1), most affected by freshwater input and with limited organic matter (OM) enrichment, Corophium orientale was the single dominant species. The most confined site (C2) was characterized by the highest levels of sedimentary OM and benthic Chlorophyll-a and by mesohaline conditions; the site was inhabited mainly by the halolimnobic Hediste diversicolor and Hydrobia spp. Site C12, characterized by a high OM load and high residence time, was dominated by the opportunistic detritivorous Alitta succinea and Polydora ciliata. At the southernmost seaward site (C3) the considerable seawater renewal, resulting in high salinity (only in summer) and limited OM load, favored a much more diverse macrobenthic assemblage, essentially composed of both marine species, such as Corophium insidiosum, Gammarus aequicauda, and brackish-water species, such as Lekanesphaera hookeri and Idotea chelipes. We conclude that the biotic and abiotic characteristics of the Cabras lagoon can be represented by a succession of spatial zones along two main gradients determined by salinity and saprobity. The salinity gradient proved to be the main structural feature in the oligohaline pole, while in the range of variable salinity, saprobity appeared to be the main selection factor. To illustrate our findings, we provide a graphical representation summarizing the changes in environmental parameters and indicator species along the salinity and saprobity gradients.
Department of Biology, University of Rome "Tor Vergata", Via della Ricerca Scientifica, 00133 Rome, Italy.
A
A
Foti
CNR-IAMC, National Research Council, Institute for Coastal Marine Environment, Località Sa Mardini, Torregrande, 09170 Oristano, Italy.
G A
GA
Fenzi
CNR-IAMC, National Research Council, Institute for Coastal Marine Environment, Località Sa Mardini, Torregrande, 09170 Oristano, Italy; CNR-IRSA, National Research Council, Water Research Institute, Area della Ricerca di Roma 1, Via Salaria km 29, Monterotondo stazione, 00015 Rome, Italy.
F
F
Di Pippo
Department of Biology, University of Rome "Tor Vergata", Via della Ricerca Scientifica, 00133 Rome, Italy.
M F
MF
Gravina
CNR-IAMC, National Research Council, Institute for Coastal Marine Environment, Località Sa Mardini, Torregrande, 09170 Oristano, Italy; CNR-ISMAR, National Research Council, Institute of Marine Science, Arsenale Tesa 104, Castello 2737/F, 30122 Venice, Italy. Electronic address: paolo.magni@cnr.it.
P
P
Magni
D063147:Animal Distribution; D000818:Animals; D058448:Biota; D004787:Environmental Pollution / Q000032:analysis*; D063366:Estuaries; D019015:Geologic Sediments; D007448:Invertebrates / Q000502:physiology*; D007558:Italy; D008522:Mediterranean Sea; D008954:Models, Biological; D054712:Salinity; D013045:Species Specificity
Brackish waters; Macrobenthos; Mediterranean lagoons; Organic enrichment; Salinity; Saprobity