Cryptic Constituents: The Paradox of High Flux-Low Concentration Components of Aquatic Ecosystems

The interface between terrestrial ecosystems and inland waters is an important link in the global carbon cycle. However, the extent to which allochthonous organic matter entering freshwater systems plays a major role in microbial and higher-trophic-level processes is under debate. Human perturbations can alter fluxes of terrestrial carbon to aquatic environments in complex ways. The biomass and production of aquatic microbes are traditionally thought to be resource limited via stoichiometric constraints such as nutrient ratios or the carbon standing stock at a given timepoint. Low concentrations of a particular constituent, however, can be strong evidence of its importance in food webs. High fluxes of a constituent are often associated with low concentrations due to high uptake rates, particularly in aquatic food webs. A focus on biomass rather than turnover can lead investigators to misconstrue dissolved organic carbon use by bacteria. By combining tracer methods with mass balance calculations, we reveal hidden patterns in aquatic ecosystems that emphasize fluxes, turnover rates, and molecular interactions. We suggest that this approach will improve forecasts of aquatic ecosystem responses to warming or altered nitrogen usage

Attached Algae: The Cryptic Base of Inverted Trophic Pyramids in Freshwaters

It seems improbable that a thin veneer of attached algae coating submerged surfaces in lakes and rivers could be the foundation of many freshwater food webs, but increasing evidence from chemical tracers supports this view. Attached algae grow on any submerged surface that receives enough light for photosynthesis, but animals often graze attached algae down to thin, barely perceptible biofilms. Algae in general are more nutritious and digestible than terrestrial plants or detritus, and attached algae are particularly harvestable, being concentrated on surfaces. Diatoms, a major component of attached algal assemblages, are especially nutritious and tolerant of heavy grazing. Algivores can track attached algal productivity over a range of spatial scales and consume a high proportion of new attached algal growth in high-light, low-nutrient ecosystems. The subsequent efficient conversion of the algae into consumer production in freshwater food webs can lead to low-producer, high-consumer biomass, patterns that Elton (1927) described as inverted trophic pyramids. Human perturbations of nutrient, sediment, and carbon loading into freshwaters and of thermal and hydrologic regimes can weaken consumer control of algae and promote nuisance attached algal blooms

Dissolved inorganic carbon sources for epipelic algal production: Sensitivity of primary production estimates to spatial and temporal distribution of \u3csup\u3e14\u3c/sup\u3eC

Estimates of epipelic algal primary production using 14C are sensitive to whether the presumptive source of dissolved inorganic carbon (DIC) is the overlying water, the interstitial water, or both. To determine the source of DIC, we compared 14C uptake among intact sediment cores exposed to different 14C ratios between interstitial and overlying water. In addition, we evaluated the effect of varying time between addition of 14C and light incubation (preincubation time) and the effect of photosynthetic uptake on final distribution of 14C. Both preincubation time and photosynthetic uptake affected final 14C distribution, but the magnitude of the photosynthesis effect was larger. Estimates of primary production ranged between 50 and 200 mg C m−2 h−1, depending on the presumed DIC source and whether we accounted for differential photosynthetic depletion of 14C and DIC. Using nonlinear regression, our best estimate of epipelic production was 114 mg C m-2 h-1, and the fraction of DIC sequestered from overlying water was 0.5 (R2 = 0.84). Similar assessments should be applied in other systems for accurate 14C estimates of epipelic algal production

Longitudinal Dynamics of Seston Concentration and Composition in a Lake Outlet Stream

Concentrations of suspended particulate organic carbon (POC), dissolved organic carbon, bacteria, and chlorophyll a were measured in a lake outlet in western Montana, USA. Seven sites within the first 3 km downstream from the lake were sampled over 14 mo. Downstream change in concentration of each variable was tested for fit to a power function (C=aDb). Downstream changes in POC, bacteria, and chlorophyll a, sometimes fit a power function, and the value of the exponent b was positively correlated with discharge. At low discharge, a downstream decline in lake algae was accompanied by an increase in stream algae. At high discharge, concentrations of lake algae did not change downstream, and some stream algae increased. These patterns suggest that although initial concentrations of suspended organic carbon (seston) are determined by the lake, within a short distance, concentrations become regulated by stream processes. As discharge increases, lake products are transported farther downstream. Thus, the extent of the influence of the lake on seston composition expands and contracts longitudinally with increases and decreases in discharge

Substratum as a Driver of Variation in Periphyton Chlorophyll and Productivity in Lakes

Quantifying periphyton (attached algal) contributions to autotrophic production in lakes is confounded by properties of substratum that affect community biomass (as chlorophyll content) and productivity. We compared chlorophyll content and productivity of natural algal communities (phytoplankton, epipelon, epilithon, epixylon, and epiphyton) experiencing high (\u3e10%) incident radiation in lakes in the US, Greenland, and Quebec, Canada. Chlorophyll content and productivity differed significantly among regions, but they also differed consistently among communities independent of region. Chlorophyll content of periphyton on hard substrata (rocks and wood) was positively related to water-column total P (TP), whereas chlorophyll content of algae on sediment (epipelon) and TP were not significantly related. Chlorophyll content was up to 100× higher on sediments than on hard substrata. Within regions, chlorophyll-specific primary productivity was highest for phytoplankton and lowest for epipelon. Periphyton on hard substrata and on macrophytes (epiphyton) had similar rates of chlorophyll-specific productivity that were intermediate to those of epipelon and phytoplankton. Area-specific productivity of epipelon was 5 to 10× higher than area-specific productivity of periphyton on hard substrata. This broad geographic comparison indicates that, in low to moderately productive lakes under high-light conditions, algal communities have predictable differences in area-specific and chlorophyll-specific productivity based on substratum. As such, chlorophyll alone is an inadequate predictor of the relative contributions of different algal communities to total primary production. Our results highlight the importance of the relative abundance and spatial distributions of substrata in determining the role of the littoral zones in nutrient and energy cycles in lakes

Attached Algae: The Cryptic Base of Inverted Trophic Pyramids in Freshwaters

It seems improbable that a thin veneer of attached algae coating submerged surfaces in lakes and rivers could be the foundation of many freshwater food webs, but increasing evidence from chemical tracers supports this view. Attached algae grow on any submerged surface that receives enough light for photosynthesis, but animals often graze attached algae down to thin, barely perceptible biofilms. Algae in general are more nutritious and digestible than terrestrial plants or detritus, and attached algae are particularly harvestable, being concentrated on surfaces. Diatoms, a major component of attached algal assemblages, are especially nutritious and tolerant of heavy grazing. Algivores can track attached algal productivity over a range of spatial scales and consume a high proportion of new attached algal growth in high-light, low-nutrient ecosystems. The subsequent efficient conversion of the algae into consumer production in freshwater food webs can lead to low-producer, high-consumer biomass, patterns that Elton (1927) described as inverted trophic pyramids. Human perturbations of nutrient, sediment, and carbon loading into freshwaters and of thermal and hydrologic regimes can weaken consumer control of algae and promote nuisance attached algal blooms

Dissolved inorganic carbon sources for epipelic algal production: Sensitivity of primary production estimates to spatial and temporal distribution of \u3csup\u3e14\u3c/sup\u3eC

Estimates of epipelic algal primary production using 14C are sensitive to whether the presumptive source of dissolved inorganic carbon (DIC) is the overlying water, the interstitial water, or both. To determine the source of DIC, we compared 14C uptake among intact sediment cores exposed to different 14C ratios between interstitial and overlying water. In addition, we evaluated the effect of varying time between addition of 14C and light incubation (preincubation time) and the effect of photosynthetic uptake on final distribution of 14C. Both preincubation time and photosynthetic uptake affected final 14C distribution, but the magnitude of the photosynthesis effect was larger. Estimates of primary production ranged between 50 and 200 mg C m−2 h−1, depending on the presumed DIC source and whether we accounted for differential photosynthetic depletion of 14C and DIC. Using nonlinear regression, our best estimate of epipelic production was 114 mg C m-2 h-1, and the fraction of DIC sequestered from overlying water was 0.5 (R2 = 0.84). Similar assessments should be applied in other systems for accurate 14C estimates of epipelic algal production

Longitudinal Dynamics of Seston Concentration and Composition in a Lake Outlet Stream

Concentrations of suspended particulate organic carbon (POC), dissolved organic carbon, bacteria, and chlorophyll a were measured in a lake outlet in western Montana, USA. Seven sites within the first 3 km downstream from the lake were sampled over 14 mo. Downstream change in concentration of each variable was tested for fit to a power function (C=aDb). Downstream changes in POC, bacteria, and chlorophyll a, sometimes fit a power function, and the value of the exponent b was positively correlated with discharge. At low discharge, a downstream decline in lake algae was accompanied by an increase in stream algae. At high discharge, concentrations of lake algae did not change downstream, and some stream algae increased. These patterns suggest that although initial concentrations of suspended organic carbon (seston) are determined by the lake, within a short distance, concentrations become regulated by stream processes. As discharge increases, lake products are transported farther downstream. Thus, the extent of the influence of the lake on seston composition expands and contracts longitudinally with increases and decreases in discharge

Substratum as a Driver of Variation in Periphyton Chlorophyll and Productivity in Lakes

Quantifying periphyton (attached algal) contributions to autotrophic production in lakes is confounded by properties of substratum that affect community biomass (as chlorophyll content) and productivity. We compared chlorophyll content and productivity of natural algal communities (phytoplankton, epipelon, epilithon, epixylon, and epiphyton) experiencing high (\u3e10%) incident radiation in lakes in the US, Greenland, and Quebec, Canada. Chlorophyll content and productivity differed significantly among regions, but they also differed consistently among communities independent of region. Chlorophyll content of periphyton on hard substrata (rocks and wood) was positively related to water-column total P (TP), whereas chlorophyll content of algae on sediment (epipelon) and TP were not significantly related. Chlorophyll content was up to 100× higher on sediments than on hard substrata. Within regions, chlorophyll-specific primary productivity was highest for phytoplankton and lowest for epipelon. Periphyton on hard substrata and on macrophytes (epiphyton) had similar rates of chlorophyll-specific productivity that were intermediate to those of epipelon and phytoplankton. Area-specific productivity of epipelon was 5 to 10× higher than area-specific productivity of periphyton on hard substrata. This broad geographic comparison indicates that, in low to moderately productive lakes under high-light conditions, algal communities have predictable differences in area-specific and chlorophyll-specific productivity based on substratum. As such, chlorophyll alone is an inadequate predictor of the relative contributions of different algal communities to total primary production. Our results highlight the importance of the relative abundance and spatial distributions of substrata in determining the role of the littoral zones in nutrient and energy cycles in lakes