Assessment of leaf litter decomposition for biomonitoring in urban watercourses under contrasting thermal conditions

Abstract

Urbanization affects the structure and function of aquatic ecosystems, and its effect might depend on seasonal conditions. We aim to evaluate the applicability of in situ leaf litter decomposition experiments to assess the ecological integrity of urbanized streams in cool (autumn–winter) and warm (spring–summer) periods. Along these two periods, three reaches were selected in urban and three in reference segments in Pampean streams. In each reach at both periods, 25 bags of 450 μm (FM) and 25 bags of 20 mm (CM) mesh size were placed containing dry leaves of Populus nigra, and five bags of each type were periodically collected up to day 104. Decomposition rates were determined from mass loss by fitting to a negative exponential model against time (kd) and cumulate degree days (kdd). In both periods, kdd were lower in urban than in reference reaches (pcondition = 0.020, df = 1, 137), but a larger difference occurred in the warm period. Even removing the effect of temperature, higher kdd were observed in warm than in cool waters (pperiod = 0.0004, df = 1, 137 for FM and pperiod = 0.002, df = 1, 129 for CM). During the warm period experiment, the kdd reduction due to urbanization was 2.5 times higher than during the cool period. Invertebrates colonizing litter bags differed between stream conditions and between seasons. Tolerant insect larvae were more abundant in the warm period; Gastropods, nematodes, and crabs during the cool period. In conclusion, our experimental methodology was effective to assess the effects of urbanization on stream ecological integrity. As we predicted, season stood out as an important factor in the assessment.