Two conclusions can be emphasized about the potential influence of chem-
ical contaminants associated with agriculture or mining. First, where they
occur, invertebrate community size structure may be shifted towards larger
taxa and/or larger individuals within taxa. Second, other environmental influ-
ences that impact on size-class structure may make individuals in the commun-
ity more, or less, susceptible to exotic contaminants.
Analysis of a new database for New Zealand streams
Having reviewed the variety of environmental gradients that can underlie
patterns in invertebrate size-class structure in streams, we now turn to a new
database of actual macroinvertebrate body sizes from 18 tributaries in the Otago
region of the South Island of New Zealand. This is a descriptive study in which
we test a variety of hypotheses derived from the review above. Our predictions
were that body size would be negatively correlated with disturbance intensity,
positively correlated with substratum grain size and substratum heterogeneity,
and positively correlated with algal productivity. We predicted that invertebrate
predator size would be negatively correlated with the presence of the voracious
brown trout and also tested the null hypothesis that the complex land-use
gradient would bear no relationship to community body-size structure.
Methods
Study sites
The tributaries and study reaches have been described in detail elsewhere
(Thompson & Townsend,2004, 2005). In brief, the study sites were 30-metre
reaches incorporating pools and riffles in second- or third-order streams with
cobble or pebble beds. Four land uses were represented: native tussock grass-
land (seven sites), improved pasture grassland (three), native podocarp forest
(two), and exotic conifer forest (six). Each site was sampled once for fish, macro-
invertebrates and a range of physicochemical factors in the summer of 1994/5
(tussock, pasture and native forest sites) or 1997/8 (pine forest sites). Two of the
tussock sites (Dempsters and Sutton) were also sampled in the spring and
autumn of 1996 to assess seasonal effects.
Fish
The presence or absence of predatory fish was ascertained by electro-fishing,
and each site was classed as having no fish, brown trout, native galaxiid fish, or
both trout and galaxiids. Note that where the fish co-occur, there are generally
very low densities and biomasses of each (Townsend,2003).
Macroinvertebrates
Ten randomly located Surber samples (area 0.06 m^2 , mesh size 250mm) were
taken at each site to sample benthic macroinvertebrates with a minimum size
BODY SIZE IN STREAMS 85