The seasonal dynamics and persistence of stream macroinvertebrates in Nepal: do monsoon floods represent disturbance?

Abstract

1. The monsoon causes major flood events in some Himalayan streams, but their seasonal predictability might reduce the resulting disturbance. We assessed seasonal change in the benthos of 16 streams in central Nepal over a gradient of declining rainfall and increasing altitude from 600 to 3800 m. All sites were surveyed on four occasions, two in winter (November) and two pre‐monsoon (June), with additional sampling during the monsoon (August) at four low altitude sites. Invertebrate abundance, taxon richness and persistence were assessed at all sites, and density and meso‐habitat distribution at the four low altitude sites only. 2. Strong seasonal variation among invertebrates was confined primarily to streams at low altitude (600–800 m) where monsoon rainfall was greatest and catchments were dominated by terraced agriculture. At these sites, a significant reduction in benthic density (on average by 77%) and taxon richness (by 20%) occurred between the winter and pre‐monsoon periods, so that invertebrate numbers were already low before the monsoon. A further significant decline occurred in all meso‐habitats during the monsoon, but the change in density was small in absolute terms. 3. Persistence in rank abundance was equally low at all sites, but turnover in composition was significantly lower at sites in semi‐natural forest than in catchments managed for terracing or alpine pasture. 4. These data provide no evidence that monsoonal floods represent major disturbance, instead supporting the view that the ecological response might reflect an adjustment to predictable flow pattern. However, catchment land use in the Himalaya appears to be a significant source of ecosystem instability, and confounds the simple interpretation of monsoon effects.

Interannual and seasonal variability of macroinvertebrates in monsoonal climate streams

“…Temporal variability in macroinvertebrate assemblage composition and structure of macroinvertebrates are strongly influenced by the monsoonal climate, as has been noted in other studies in different regions of the world (Jacobsen and Encalada 1998;Brewin 2000;Bogan and Lyttle 2007). In Lules River basin, seasonal variability showed significant differences in the environmental variables, influencing strongly the community parameters.…”

“…High discharge events can cause severe population losses and changes in the community composition and structure (Hart and Finelli 1999;Lytle et al 2008). Streams in monsoonal environments are subject to extreme seasonal variation in flow (Jacobsen and Encalada 1998;Brewin et al 2000). Floods and spates in these areas are qualified as disturbances due to their large magnitude and geomorphological effect, but they are highly predictable (Resh et al 1988).…”

Seasonal persistence and stability of diatom communities in rivers: are there habitat specific differences?

“…Furthermore, changes in species composition or percentage turnover (T) were used to indicate community persistence. T was calculated as T = (G + L)/ (S 1 + S 2 ) 6 100 where G and L are the number of taxa gained and lost between months, and S 1 and S 2 are the number of taxa present in successive sampling months (Diamond & May, 1977;Brewin et al, 2000). Dominance and species richness of diatom assemblages on each substratum were evaluated using rank-abundance diagrams.…”

“…The persistence and stability of lotic communities, mainly containing macroinvertebrates and fishes, have been studied extensively (Moyle & Vondracek, 1985;Townsend et al, 1987;Weatherley & Ormerod, 1990;Brewin et al, 2000;Scarsbrook, 2002;Woodward et al, 2002). These studies span several years, even decades (Woodward et al, 2002).…”

Temporal variations in macroinvertebrate communities from the tributaries in the Three Gorges Reservoir Catchment, China

“…However, in our study, temporal variations in macroinvertebrate communities were still detected at the family level. Strong seasonal variation among macroinvertebrates was confined primarily to streams at low altitudes (600-800 m) with the greatest monsoon rainfall [51].…”