Effects of phosphorus enrichment on detritus and detritivores in Ozark streams

Background/Question/Methods

Field studies examining the impacts of nutrient enrichment on detrital-based stream communities have provided evidence that reductions in detrital resource quality [carbon:phosphorus (C:P)] and quantity can result in shifts in macroinvertebrate community structure and function, but the relative importance of quality and quantity to this shift has not been addressed. We are using field and laboratory studies to determine whether observed threshold reductions in detritivorous insect diversity with increasing total P (TP) concentration in the Ozark Highland streams can be caused by changes in detrital resource quality (C:P).  We examined the effect of added P concentration (0, 50, and 500µg/L) on dominant Ozark tree species (maple and oak) leaf litter quality in 139 d laboratory incubations.  We expected leaf litter %P would saturate at higher levels with increasing P concentration and in oak versus maple.  In addition, we quantified coarse benthic organic matter (CBOM) quantity and quality and shredding insect abundance, biomass, and C:P across streams (n=12) with differing TP concentrations (range=8-62µg/L) and expected to find negative relationships between CBOM C:P and shredders with lower body C:P.  We expected that higher body C:P shredders would have either no relationship or a positive relationship with CBOM C:P.

Results/Conclusions

Average oak and maple litter %P (range) in the no additional P incubation were 0.06 (0.04-0.09) and 0.05 (0.033-0.072), respectively, and did not change over time.  Saturating %P levels over time were estimated to be 0.25 (p<0.001) and 0.41 (p<0.001) for maple and 0.08 (p=0.010) and 0.14 (p=0.006) for oak litter in the 50 and 500 µg/L added P incubations, respectively.  Stream CBOM C:P was positively related to TP concentration (p < 0.01, R² = 0.56).  No statistically significant relationship was found between CBOM standing stocks and stream TP concentrations.  Higher C:P shredders (Strophopteryx sp. and Amphinemura sp.) were positively or not related to CBOM C:P.  Lower C:P shredders (Pychnopsyche sp. and Ephemerella sp.) were negatively related to CBOM C:P or positively to TP concentrations with the exception of Tipula.  Shredding insect abundance and biomass were not related to CBOM standing stocks.    Future experiments will quantify threshold elemental C:P ratios for Ozark shredding insects in the laboratory to determine if causal relationships between detrital C:P and  insect growth and survivorship exist that could be used to predict nutrient concentrations where particular species decline in Ozark streams.