The importance of excretion by Chironomus larvae on the internal loads of nitrogen and phosphorus in a small eutrophic urban reservoir

R Henry,Cm Santos

doi:10.1590/s1519-69842008000200016

Abstract

Measurements of ammonium and phosphate excretion by the Chironomus larvae were conducted in order to evaluate the importance of these chironomids for the internal loads of a small eutrophic urban reservoir. Ammonium and phosphate excretion rates by Chironomus larvae of small size (6-10 mm total length) were significantly higher than those of the Chironomids having medium (9-11 mm) and large (11-16 mm) sizes. A dependence in relation to temperature was recorded for the ammonium and phosphate excretions that was significantly higher at 25 degrees C than at 20 and 15 degrees C. Through a linear relation between biomass (dry weight) and total length and, between excretion and biomass and, data on chironomids densities, after an intense sampling in 33 sites distributed all along the reservoir bottom, the mean phosphate and ammonium excretion rates corresponded to 2,014 +/- 5,134 microg.m(-2)/day and 1,643 +/- 3,974 microg.m(-2)/day, respectively. Considering the mean biomass (34 mg.m(-2)) of Chironomus, the lake area (88,156 m(2)) and the mean excretion rates, the contribution of benthic chironomids to the internal loads would be 181 KgP and 147 KgN. for the sampling months (October-November 1998). These values showed that the internal loads by excretion from Chironomus larvae correspond to approximately 33% of the external loads of phosphorus in the lake and, in the case of nitrogen, to only 5%.

Highlights

Benthic invertebrates play an important role in the ecology of aquatic ecosystems
Benthic species release nutrients that are dissolved by their feeding activities, excretion and burrowing into sediments (Covich et al, 1999; Vanni, 2002)
This paper reports on a quantitative analysis of ammonium and phosphate excretion rates by Chironomus and discusses the importance of chironomid assemblies in the internal loads of nitrogen and phosphorus of a small urban reservoir (Lake Garças, São Paulo, Brazil)

Summary

Introduction

Benthic invertebrates play an important role in the ecology of aquatic ecosystems. Bottom organisms increase the velocity of decomposition of particulate matter, breaking up the litter of riparian vegetation in lotic environments and of aquatic macrophytes in the littoral zone of lacustrine ecosystems (Stripari and Henry, 2002; Henry and Stripari, 2005). Benthic species release nutrients that are dissolved by their feeding activities, excretion and burrowing into sediments (Covich et al, 1999; Vanni, 2002). Benthic fauna are predators that control the density and size of their prey and are a source of food for other consumers such as terrestrial and aquatic vertebrates, namely birds and fish. Benthic invertebrates accelerate the transfer process of nutrients from interstitial.

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