Combining gut fluorescence technique and spatial analysis to determine Littorina littorea grazing dynamics in nutrient-enriched and nutrient-unenriched littoral mesocosms
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Diaz ER, Krauvelin P, Erlandsson J
mesocosm, land based, littorina, 12 m3, Solbergstrand, Oslofjord, Norway
Spatiotemporal distribution patterns in relationto feeding behavior of herbivorous gastropods have beenstudied extensively, but still knowledge about small-scalepatterns is limited in relation to eutrophication. Thisexperimental study aimed to describe the small-scale distributionof Littorina littorea in nutrient-enriched andnutrient-unenriched mesocosms in a merely atidal regionand relate the distribution to food abundance and possiblecompeting organisms, while checking simultaneously forfeeding activities. The latter part was accomplishedthrough the ‘‘gut fluorescence technique’’ GFT (which, toour knowledge, has not previously been used for benthicgrazers) to estimate per capita grazing rates and the formerpart through monitoring of spatial heterogeneity of L. littoreaand co-variation with sessile organisms (usingsemivariograms and cross-semivariograms, respectively).After 5 months of nutrient addition, the abundance andbiomass of L. littorea had increased in enriched systems,which also had significantly higher total biomass of greenalgae. Gut pigment content was higher in L. littorea fromenriched mesocosms, and gut depletion rate was higher inL. littorea from unenriched mesocosms. Spatial analysisshowed that L. littorea exhibited generally random patterns(suggesting feeding activities) but sometimes (often in themorning) spatial patchiness (clumped distribution) in bothenriched and unenriched conditions. There was mainlypositive co-variation between L. littorea and biofilm, whiledifferent nutrient conditions exhibited contrasting co-variationbetween L. littorea and barnacles (positive co-variationin enriched and negative co-variation in unenrichedmesocosms). The study offered insights into how feedingbehavior and spatial distribution of a species may interactwith community components differently under differentnutrient regimes. The applied methodology can be usefulfor purposes of faster examination of grazing effectsamong different regions and also to compare grazingintensities and interactions between grazers and the benthiccommunities in disturbed (including pollution and nutrientenrichment) and non-disturbed systems, as well as in upwellingversus non-upwelling areas.
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