The roles of local disturbance history and microhabitat parameters for stream biota

The identification of factors that cause the patchy distribution of
organisms in space and time within natural ecosystems is a central
concern in ecology. In running waters, disturbance by bed-moving
flows has been recognized to contribute to this patchiness, but the
mechanisms behind this process are still poorly understood. Recent
research has revealed that most bed-moving spates and floods cause
a small-scale mosaic of stream bed patches of different substratum
stabilities (sediment scour, sediment deposition, and stable
patches). The aim of the present research was to investigate the
separate and combined roles of this “local disturbance history” and
microhabitat parameters in determining the small-scale
distributions of benthic stream organisms (bacteria, algae and
invertebrates). The first three chapters of my thesis investigate
bacteria, algae and invertebrates after natural spates and floods
in two gravel-bed streams, the Schmiedlaine in Bavaria, Germany,
and the Kye Burn in the South Island of New Zealand. The final
chapter describes a manipulative experiment in the Schmiedlaine in
which the stream bed was disturbed manually to obtain a better
understanding of how disturbance history affects stream organisms.
Local bed movement patterns during several natural disturbances
were determined using metal-link scour chains. The same chains were
also used as reference points when selecting stream bed patches for
manipulation during the experimental disturbance. Quantitative
samples were collected several times from random sites before each
disturbance and from scour, fill and stable patches after each
disturbance. In addition to sampling bacteria, algae and
invertebrates, we measured primary production under controlled
conditions in both streams and bacterial production in the Kye
Burn. For each sampling site, we determined several abiotic and
biotic microhabitat parameters that are known to influence the
distributions of stream biota, e.g. water depth, substratum
composition, near-bed current velocity and standing stock of
particulate organic matter (POM). The results of the correlative
studies suggest that local disturbance history plays an important
role in determining the small-scale distributions of stream
bacteria, algae and invertebrates. Disturbance history was related
to distributions of all three organism groups both directly and
indirectly (via effects on habitat parameters) and significant
habitat parameter effects revealed previously undetected history
effects in several cases, indicating a strong interaction between
disturbance history and the better explored habitat parameters.
Bacterial microdistribution was related directly to disturbance
history at least as often as to any of the other nine studied
habitat parameters (except for the surface area of the sampled
stones in the Schmiedlaine). The relationships of local disturbance
history with algal distributions were not quite as strong. In the
flood-prone Schmiedlaine, substratum characteristics and current
velocity were related to algal distribution at least as often as
disturbance history. In the relatively more stable Kye Burn, there
were no direct influences of disturbance history on the algae.
Substratum characteristics, water depth, current velocity and POM
were the habitat parameters with the highest numbers of significant
relationships. However, disturbance history also influenced algal
distribution indirectly in both streams, and this indirect
influence was particularly strong in the Kye Burn. Primary
production was always similar across bed stability types in both
streams. Averaged across both streams, invertebrate distribution
was related only to near-bed current velocity more often than to
disturbance history. In the Kye Burn, disturbance history and water
depth were the habitat parameters that were related most often to
invertebrate distributions. Further important parameters for
invertebrates were substratum size and epilithic algal biomass. The
observed relationships of benthic densities to the investigated
habitat parameters were largely in accordance with previous
microhabitat studies for benthic algae and invertebrates. These
results also show that benthic bacteria in streams appear to be
influenced by similar habitat parameters as benthic algae and
invertebrates. The findings of the experimental disturbance in the
Schmiedlaine generally supported those of the correlative research,
lending more weight to the generality of the results. Local
disturbance history influenced directly algal microdistribution in
the stream bed most often, followed by near-bed current velocity,
total invertebrate density and the surface area of the sampled
stone. Primary production was also always similar between the three
bed stability types. In contrast to the benthic algae and the
correlative study on bacteria, bacterial microdistribution in the
experiment was influenced directly solely by stone surface area.
However, disturbance history had a strong indirect influence via
habitat parameters on bacterial distribution. For invertebrate
distribution, disturbance history was the parameter with the
second-highest number of significant relationships after current
velocity. Indirect effects of disturbance history on the stream
organisms were similarly common as in the correlative studies and
significant habitat parameter effects revealed previously
undetected history effects in several cases, confirming the strong
interaction between disturbance history and the better- explored
habitat parameters. The combined findings of the present research
provide substantial evidence that local disturbance history is an
important, and previously unrecognised factor contributing to the
patchy distribution of stream organisms in space and time.