Testing effects of detritus input and export on macroinvertebrate communities in tropical headwater streams

Background/Question/Methods

Autumn leaf fall is an important source of detritus for temperate stream invertebrates, and seasonal patterns in community structure is strongly determined by changes in detrital inputs.   Few studies have examined this factor in tropical aquatic systems, which have relatively constant detrital inputs due to lack of autumn leaf fall.  However, seasonal changes may occur when detritus is washed out by spates during the rainy season. I tested the effects of detrital resource changes on macroinvertebrate community structure in first order tributaries of the San Lorencito River in central Costa Rica. I hypothesized that invertebrate communities would be affected by spates and experimental manipulation of detrital inputs. I also hypothesized that macroinvertebrate responses would differ in pool, riffle and run microhabitats. 24 stream reaches (10-m long) were randomized into three treatments: 1) a one-time manual detritus removal and continuous leaf exclusion with overhead netting 2) one time manual detritus removal without netting and 3) an unmanipulated control.  Invertebrate communities in pool, riffle and run microhabitats within each site were sampled in April, May, June and August 2012. Samples were collected by disturbing the stream bed substrate and allowing dislodged materials to flow into an 800um sieve.

Results/Conclusions

Preliminary results are available from the entire June sample set as well as April, June and August control treatments. NMS ordination and Multi-response Permutation Procedure (MRPP) analyses showed that communities in June differed by treatment (p<0.01), stream (p<0.01) and site (p<0.01)  indicating that several factors affect macroinvertebrate community structure. ANOVA tests found total numbers were not different among microhabitats or between leaf exclusion treatments.  However, total numbers without Chironomidae, which comprised 60% of all invertebrates, were significantly different between treatments   (p=0.016).  Cluster analysis of invertebrate communities in control sites for dates transitioning into the rainy season (April, June, August) suggests that communities in pools are more resistant to spates then those in riffles. Overall, results show that invertebrate communities are heterogeneous between the microhabitats of these headwater streams and that there are pronounced seasonal changes that may be due to the impact of spates. I found less of an effect of manipulating detrital inputs than expected, but this is likely due to the ineffectiveness of the enclosures to keep out detritus smaller then 2cm.