Temporal and spatial patterns in stream physicochemistry and insect assemblages in a tropical lowland streams.
Abstract: Stream physicochemistry and insect assemblages were studied in lowland
tropical streams in Costa Rica to determine physicochemical variables
explaining temporal and spatial variations in insect density and
biomass. Streams drained a tropical wet forest landscape and had
similar geomorphology, but differed in their water chemistry because of
differential inputs of solute-rich (i.e., P, Cl–, and Mg)
geothermally modified groundwater. We hypothesized that, within a
stream, physical factors, such as discharge, would result in temporal
variation in insect assemblages, whereas interstream differences in
solute composition would result in differences in insect assemblages
among streams. Insect density, biomass, richness, and stream
physicochemistry were measured monthly for 1 y in 6 streams. Insect
samples were collected with a core sampler in runs where leaf litter
overlying fine sediments was the main substrate. Streams with high
inputs of geothermally modified groundwater had high conductivity and
high concentrations of soluble reactive P (SRP). Annual temporal
patterns in stream physicochemistry were related either to rainfall,
with subsequent changes in discharge, or to the % geothermally modified
groundwater entering streams. Streamwater pH decreased throughout the
year, and unbuffered, nongeothermally modified streams changed from
near neutral (pH >6) to near acidic (pH <4.5). In all streams,
insect density and biomass were highest during the dry season and
lowest during the wet season. Insect density and biomass were related
positively to pH and to the number of days since the last storm. Insect
taxonomic composition was similar among streams. Moreover, interstream
variation in insect assemblages was small, and insect density and
biomass were not related to any of the measured physicochemical
variables, including SRP, which ranged from <5 to 267 μg/L. Overall,
the physicochemical characteristics of the study streams were related
mainly to geothermally modified groundwater inputs and seasonal
patterns in rainfall. Streams with geothermally modified groundwater
inputs were better buffered against pH changes than the other streams.
Insect assemblages reflected fluctuations in stream physicochemistry
during the year and were associated with 2 main factors: floods and pH.
Physicochemical characteristics were very different among streams, but
insect assemblages were not related to them. We hypothesize that the
heterogeneous nature of benthic substrata in the streams resulted in
high variation in density and biomass of insects, potentially obscuring
differences in insect assemblages among streams.
Ramirez, A. , C. M. Pringle, and M. Douglas. 2006. Temporal and spatial patterns in stream physicochemistry and insect assemblages in a tropical lowland streams. Journal of the North American Benthological Society 25: 108-123.
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