Benthic respiration

Benthic respiration is the integrative measure of stream metabolism and C utilization from all sources. It includes respiration by primary producers, microbial heterotrophs, and animals, which as mentioned earlier can be visualized as RA of autotrophs and RH of heterotrophs. Respiration by microorganisms often is the largest component of RH, reflecting the important roles of bacteria and fungi in the breakdown of organic matter and their ability to utilize labile DOM from streamwater. Because metabolic processes are strongly temperature dependent, respiration is expected to vary with temperature, as Bott et al. (1984) reported for streams in Michigan and Pennsylvania, and in general to be strongly seasonal. Total respiration should increase with increasing amounts of benthic organic matter (BOM), but the quality of BOM is at least as important as quantity (Findlay et al. 1986a). In a Tennessee woodland stream, respiration was highest in early spring due to high GPP and infall after leaf input, which are periods of moderate temperature, and low during the warmer midsummer period because of low organic matter supplies (Roberts et al. 2007). A downstream increase in benthic respiration might be expected if total C inputs increase, because warmer temperatures stimulate higher rates, or because larger rivers receive greater inputs from domestic sewage or agricultural runoff. Due to the scarcity of data for large rivers, changes in R with increasing river size are poorly documented. However, downstream increases in R have been reported in systems as disparate as the highly autotrophic Salmon River (Minshall et al. 1992) and highly heterotrophic blackwater rivers in Georgia (Meyer and Edwards 1990).

In a cross-biome comparison of 22 streams, Sinsabaugh et al. (1997) analyzed stream benthic respiration rates in relation to BOM, temperature, primary production, and other system variables. Benthic respiration was directly proportional to stream temperature and, presumably due to high rates of utilization, the standing stock of BOM was inversely related to stream temperature (Figure 12.7). Owing to these offsetting trends, specific respiration per gram of benthic organic C was strongly related to temperature. Because the coefficient of this relationship was too high for a simple metabolic response, Sinsabaugh inferred that other factors also must be operating, such as higher quality BOM or greater nutrient availability in streams of

FIGURE 12.7 Relationships of respiration rate and standing stock of benthic organic carbon (BOC) with stream temperature for 22 streams. (a) BOC decreases and (b) respiration rate increases with mean annual water temperature. (c) Specific respiration increases and (d) the turnover time of BOC decreases with increasing temperature. See text for further explanation. (Reproduced from Sinsabaugh et al. 1997.)

FIGURE 12.7 Relationships of respiration rate and standing stock of benthic organic carbon (BOC) with stream temperature for 22 streams. (a) BOC decreases and (b) respiration rate increases with mean annual water temperature. (c) Specific respiration increases and (d) the turnover time of BOC decreases with increasing temperature. See text for further explanation. (Reproduced from Sinsabaugh et al. 1997.)

warmer climates. The mean respiration rate of 265 g C m2 year1 calculated from the data set analyzed by Sinsabaugh et al. (1997) was considerably lower than the average of 739 g C m 2 year1 calculated by Webster et al. (1995) from 61 streams of the eastern deciduous biome of the United States. Differences may be partly attributable to methods and partly to stream type and climate. Neither synthesis found a clear influence of stream order, and it may be that longitudinal studies of a single system rather than cross-system comparisons of streams are better suited to the investigation of downstream trends.

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