Primary production by benthic algae

Autotroph primary production is measured by gas exchange or14 C uptake in benthic chambers or by open stream gas exchange. Within closed benthic chambers, a stone with benthic algae or an enclosed area of substrate is incubated over time, so that both photosynthesis and respiration influence changes in oxygen levels during the day, whereas only respiration takes place at night. A pump circulates the water within the chamber to simulate current. Respiration (R) is estimated from the decline in oxygen concentrations that occurs in the dark, based on night measurements. Net primary production (NPP), which is the difference between photosynthetic release of oxygen and its consumption by respiration, is estimated from any change in oxygen during the day. Gross primary production (GPP) due to photosynthesis is the sum of R, estimated from night measurements, and NPP, estimated from day measurements (Bunn et al. 1999, Bott 2006).

Open stream gas exchange uses 24 h changes in O2 within the stream itself to estimate NPP (during the day) and respiration (during the night), and so has the advantage of estimating rates of photosynthesis and respiration for the whole ecosystem (Lamberti and Steinman 1997).

This method includes not only the primary production of benthic algae, but from all submerged autotrophs in the stream. Originally applied to a highly productive spring (Odum 1956), the challenge is to accurately measure changes in oxygen concentrations due to biological activity in systems where production may be low and gas exchange due to turbulence is high. Methods to estimate diffusion between air and water include the use of upstream and downstream oxygen recorders to measure changes in oxygen concentration along the reach, and direct measurement of the reaeration coefficient by injecting a gas (propane) into the water column and measuring its release to the atmosphere (Marzolf et al. 1994, Mulholland et al. 2001). The single-station method is preferred for sites with low turbulence and high primary production while the two-station method is more accurate in fast-flowing water with low primary production (Young and Huryn 1998).

Both methods pose technical difficulties and require assumptions that are difficult to check. Nighttime oxygen fluxes are assumed to represent 24 h respiration. Respiration rates include microbes and other organisms in addition to the algae, which complicates the interpretation of GPP and NPP (discussed further in Section 12.4.2). In using benthic chambers, care must be taken to simulate current and irradiance appropriately and to avoid unnatural changes in nutrient and gas concentrations, including the formation of bubbles (Bott et al. 1997). In addition, benthic chambers can only be used with substrate of a certain size range, raising questions concerning how well they represent average conditions (Bott 2006).

Although not currently in use for stream benthic algae, the pulse-amplitude modulating (PAM) fluorometer has the potential to provide a noninvasive and compact method to measure photosynthetic rates in streams. Fluorescence correlates well with CO2 uptake in higher plants, although for algae the relationship appears more variable (Gilbert et al. 2000, Jakob et al. 2005).

However, the recent demonstration that fluorescence correlates well with photosynthesis measured under light limitation and nutrient saturation conditions for periphyton and phyto-plankton suggests that fluorescence has the potential to become more widely used to estimate primary production in the future (Toepel et al. 2004).

Estimates of benthic algae primary production are relatively few, owing to the difficulty and expense of its measurement. Tentatively, it appears that maximal daily NPP in deciduous biome studies ranges from below 0.01 to ~ 0.1g C m2 day1 in shaded areas (Elwood and Nelson 1972, Hornick et al. 1981, Minshall 1967, Acuna et al. 2004), whereas values from 0.25 to ~ 2g C m2 day1 are common at sites where the canopy is open (Berrie 1972a, Bott 1983, Hill and Webster 1982, McDiffett et al. 1972, King and Ball 1966, Sumner and Fisher 1979, Mulholland et al. 2001). These values are consistent with estimates from laboratory streams provided with adequate light, where GPP was between 2.4 and 6g C m2 day1, and NPP typically was slightly more than one half of GPP (McIntire and Phinney 1965). Estimates from streams flowing through grasslands (Prophet and Ransom 1974), in arid regions (Minshall 1978, Fisher et al. 1982, Molla et al. 1996, Mulholland et al. 2001, Velasco et al. 2003), and open coniferous biome streams (Thomas and O'Connell 1966, Wright and Mills 1967) appear to give maximal values between 1 and perhaps 18gCm2day 1 in systems receiving ample sunlight. Some very productive flowing water systems are reported to achieve daily GPP approaching 20 g C m 2 day1 (e.g., Odum 1956, Duffer and Dorris 1966, Mulholland et al. 2001). Thus, estimates of daily primary production vary among sites, seasons, biomes, and climatic regions by as much as three orders of magnitude (Lamberti and Steinman 1997).

The river continuum concept (RCC) predicts that primary production will be lowest in forested headwaters, increase in more open, midsized rivers, and decline in turbid, higher-order segments (Vannote et al. 1980). This expectation is supported by higher estimates of primary production in unshaded as compared to shaded stream reaches, as just described. Estimates of GPP for 30 streams of order 1-7 located mainly in the northern hemisphere found that catchment area was the single best predictor, presumably because production responded to higher light levels and nutrient concentrations as rivers increased in size (Lamberti and Steinman 1997). There seems little doubt that periphyton production will decline in large, turbid rivers, because scant light reaches the river bed and stable substrate may be lacking as well. However, phyto-plankton and macrophyte production tend to increase in the downstream direction, and so it is difficult to say how overall system production varies across rivers of order >7.

An ambitious effort to evaluate the RCC measured primary production over a range of stream sizes in four distinct biomes of the United States (Minshall et al. 1983). Each stream system was located in a relatively undisturbed catchment, had as its uppermost station a forested headwater site, and took for its lowermost station the largest stream site that was relatively undisturbed (seventh order). Oregon sites receive abundant precipitation, mostly as winter rain, and support dense conifer forests. Idaho sites lie in a cold arid region of the northern Rocky Mountains, where forest cover is less than other biomes and runoff is dominated by melt of the winter snowpack. Coniferous forest and scattered deciduous tree species are found at upper elevations, and sagebrush or grass vegetation at lower elevations. Michigan and Pennsylvania sites are in the eastern deciduous biome, with less-pronounced seasonality in precipitation and runoff than either of the western sites. Observed variation in the standing crop of periphyton among sites, seasons, and locations was consistent with what is known of the various factors that limit algae. GPP was lowest at the heavily forested Oregon sites, usually increased in the downstream direction, and was lowest in winter.

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