Reversal of density dependence of juvenile Littorina littorea (Gastropoda)growth in response to periphyton nutrient status

Experimental periphyton communities were frown in aquaria receiving media o f differently enriched seawater (fully enriched, without Si enrichment, wit hout N and P enrichment) and supplied differently with medium (batch and we ekly replacement), Periphyton was subject to grazing by 1-6 individuals of juvenile Littorina littorea. Periphyton biomass was higher in the replaceme nt aquaria than in the batch aquaria and higher in the full and the -Si med ium than in the -NP medium. The N:C ratio of the periphyton increased with Littorina number in the batch aquaria and was unaffected by Littorina numbe r in the replacement aquaria. Diatoms were most dominant in the -NP treatme nts and rarest in the -Si treatments. Chlorophytes were dominant in the -Si and the fully enriched treatments, but also Cyanobacteria contributed sign ificantly to periphyton biomass in those treatments under nutrient replacem ent. Somatic growth of Littorina was negatively correlated to Littorina den sity in the replacement aquaria and positively density dependent in the bat ch aquaria. The latter is explained by improved food quality under stronger grazing pressure. (C) 2001 Elsevier Science B.V. All rights reserved.