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Nutrient recycling - the ideal way to avoid nutrient limitation in a grazer community

January 17, 2000

The rocky shore of Lake Erken, Sweden, is inhabited by the sessile psychomyiid caddisfly Tinodes waeneri (L.). The larva lives in a gallery, consisting of a spun silken web plus other organic and inorganic material associated with the web (Danecker 1961, Becker 1993, Hasselrot 1993a). The additional organic material includes detritus and living microorganisms (bacteria, fungi, and periphytic algae). Periphytic algae represent  90 % of the organic biomass associated with the silken web (M. Kahlert, unpublished data). The larva and gallery together form the Tinodes community. The larva feeds by scraping microorganisms from the gallery's walls and floor. It also eats parts of the silk web including the microorganisms, and then repairs the web (Danecker 1961, Becker 1990, 1993, Hasselrot 1993a, 1993b).
The Tinodes community covers 10-20 % of the rock surface in shallow, wave-exposed habitats of Lake Erken (Hasselrot 1993b). Population density is even higher in Lake Esrom, Denmark, where T. waeneri covers at least
30 % of the favorable habitats, and in extreme cases even 100 % (Dall et al. 1984, Dall et al. 1990). This fairly dense T. waeneri population in Lake Erken is of interest because epilithon growing at the same is nutrient limited throughout the entire year. Phosphorus limitation is moderate and apparent only in summer, but N limitation is severe (Kahlert 1998).
Free-living macroinvertebrates such as snails excrete a large amount of ingested N in the form of NH4, which can serve as a N source for the surrounding epilithic community in some instances (e.g., Grimm 1988). Nitrogen regeneration in turn could lead to an increase in gross primary production of periphyton under moderate grazing (Allan 1995). Yet, most benthic grazers are rather loosely associated with their food. A close association between grazers and microorganisms would minimize leakage of excretion products and feces, and potentially enhance nutrient recycling.
There is evidence that a closed, coupled grazer/microbial community is less nutrient limited than periphyton in general because of more efficient use of nutrients. Investigations by Hershey (1988) on the communities of tube-building chironomids and by Hasselrot (1993b) on the Tinodes community in Lake Erken both documented high larval densities and higher algal biomass on the tubes than in the surrounding epilithon. The benefit was regarded as mutual. The algae profited by the high N and P concentrations in the tubes because of larval excretion, and the larvae benefited from the occurrence of high algal biomass.
Nutrient recycling by the Tinodes community is a valuable adaptation maintaining a high level of productivity in the wave-exposed, N-limited littoral zone where nutrients are readily delivered and removed. Population densities of 1000 - 2000 individuals per m2 in all wave-exposed rocky habitats from 0 to 2.5 m depth in Lake Erken (Hasselrot 1993b) and densities up to 11,500 individuals per m2 in Lake Esrom, Denmark (Dall et al. 1984), indicate the importance of enhanced N recycling by the Tinodes community for a rocky shore ecosystem. Another fact might demonstrate the good adaptation of the Tinodes community: The caddisfly is the most abundant species in the stony littoral of Danish lakes (Brodersen et al. 1998)

Uppsala Universitet