Secondary production of a stream metazoan community: Does the melofauna make a difference?

The benthic communities of streams contain invertebrates of a wide range of body size and from many taxa. Owing mainly to methodological problems, however, the contribution of smaller and more obscure metazoans to community structure and dynamics, including production, is poorly known compared with that of larger size fractions and, particularly, insects and macrocrustaceans. Based on a monthly survey of a first-order, acidic English stream, we used the size-frequency method to estimate annual production of the whole metazoan benthos (down to organisms retained on a 42-mum mesh and being as taxonomically inclusive as possible). Mean total secondary production (5.22 g dry weight m(-2) yr(-1)) was low, presumably mainly due to the stream's acidity. About 15% (0.76 g m(-2) yr-1) of this total production, however, was contributed by the permanent meiofauna (species always small enough to pass through a 500-mum mesh, and mainly made up of ostracods and copepods but also including rotifers, microturbellarians, and others). By estimating separately production from the macrofaunal and meiofaunal net fractions (500 mum and 42-500 mum, respectively), we found that about 51% of total production could be accounted for by the permanent and temporary meiofauna together, the latter being defined as organisms that potentially grow into the macrofaunal size class and here consisting primarily of oligochaetes, chironomids, and plecopterans. This study points to the potentially substantial underestimation of production arising from the problems of assessing the meiofauna, including the former use of coarse-meshed (e.g., 500 mum) sampling devices, the requirement for live sorting of many soft-bodied taxa, and other difficulties of counting and identifying less well known groups.