Density, biomass, and production of selected benthic invertebrate taxa in two natural lakes and one fertilized lake in insular Newfoundland

Abstract

Most studies of experimental perturbation of the benthiccommunity employ simple estimates of numerical abundancewhen making comparisons among treatments (Rosenberg& Resh 1993, Benke 1994). Although valid in manyinstances, numerical abundance is influenced by the balancebetween reproduction and death, each of which may respondindependently to the perturbation. Simple numerical esti-mates also cannot reflect differences in individual size thatmay result from growth rate enhancement. Secondary pro-duction estimates take reproduction, growth, and death intoaccount (Benke 1994) and thus may be more powerful indi-cators of ecosystem change and give us a better understand-ing of why changes are occurring. Secondary production esti-mates are infrequently reported, probably because theyrequire large sample sizes and labour-intensive weight deter-minations.Thus, benthic community production has usually beenneglected in mesocosm nutrient enrichment experiments andbenthic production estimates are almost non-existent inwhole-lake nutrient enrichment experiments. When benthicproduction estimates have been reported with respect toexperimental nutrient additions the results have been con-flicting. Peterson et al.(1993) observed initial growth rateincreases, but total secondary production did not increase dueto “top-down” processes (predation) in a tundra river system.Aagaard (1982) reported density increases for 2 chirono-mid species during a whole-lake enrichment experiment, butthese increases and the production estimates were reported asbeing within the limits expected by annual variation alone,and thus the effect due to the nutrient enrichment was notclear.Our study had 2 main objectives. First, to document thebenthic macroinvertbrate biomass and production of thedominate taxa in 3 small Newfoundland lakes and to put thisinformation in context with respect to other estimates foundin the literature. To our knowledge this is the first attempt atcalculating secondary production estimates for these habitatsin Newfoundland despite them being common and importantto the overall production of highly valued salmonid species(Dempson et al. 1996).Our second objective was to document the relationshipbetween whole-lake nutrient enrichment and its effect onbenthic secondary production. The working hypothesis forthis objective was that the increase in nutrient load to theenriched ecosystem would increase primary productivity,which in turn would increase the productivity of the benthos.Benthic responses could be manifested as numericalincreases (Clarke et al. 1997) and/or growth rate increases(larger size), both of which should be reflected in populationproduction estimates.