Heterotrophic bacteria are thought to be important components of aquatic ecosystems in several ways.These bacteria remineralize organic materials and convert some organic material into bacterial biomass.We examined data from 70 studies in which estimates of production of heterotrophic bacterial biomass (bacterial production) were reported for fresh-and saltwater ecosystems.In sediments, bacterial production was sigdicantly (p <0.001), positively correlated to sediment organic C content.Systems which had hlgh rates of benthic primary production (such as coral reefs) had rates of bacterial production greater than those predicted by sediment organic C content alone.In the photic zone of lakes and the ocean, bacterial production was significantly correlated with planktonic primary production, chlorophyll a, or numbers of planktonic bacteria.For all planktonic systems analysed, bacterial production ranged from 0.4 to 150 pg C 1-' d-' and averaged 20 % (median 16.5 %) of planktonic primary production.On an area1 basis for the entire water column, bacterial production ranged from 118 to 2439 mg m-2 d-' and averaged 30 % (median 27 %) of water column primary production.Heterotrophic bacterial production is, thus, a large component of total secondary production and is roughly twice as large as the production of macrozooplankton for a given level of primary production.
Abstract Predicting algal blooms both within and among aquatic ecosystems is important yet difficult because multiple factors promote and suppress blooms. Statistical indicators (e.g., variance and autocorrelation) based on time series can provide warning of transitions in diverse complex systems, including shifts from clear water to algal blooms. Analogous spatial indicators have been demonstrated with models and empirical data from vegetated terrestrial ecosystems. Here, we test the applicability of spatial indicators to algal blooms using a nutrient‐phytoplankton spatial model. We found that standard deviation and autocorrelation successfully distinguished bloom state and proximity to transitions, while skewness and kurtosis were more ambiguous. Our findings suggest certain spatial indicators are applicable to aquatic ecosystems despite dynamic physical–biological interactions that could reduce detectable signals. The growing capacity to collect spatial data on algal biomass presents an exciting opportunity for application and testing of spatial indicators to the study and management of blooms.
Salt pollution and human-accelerated weathering are shifting the chemical composition of major ions in fresh water and increasing salinization and alkalinization across North America. We propose a concept, the freshwater salinization syndrome, which links salinization and alkalinization processes. This syndrome manifests as concurrent trends in specific conductance, pH, alkalinity, and base cations. Although individual trends can vary in strength, changes in salinization and alkalinization have affected 37% and 90%, respectively, of the drainage area of the contiguous United States over the past century. Across 232 United States Geological Survey (USGS) monitoring sites, 66% of stream and river sites showed a statistical increase in pH, which often began decades before acid rain regulations. The syndrome is most prominent in the densely populated eastern and midwestern United States, where salinity and alkalinity have increased most rapidly. The syndrome is caused by salt pollution (e.g., road deicers, irrigation runoff, sewage, potash), accelerated weathering and soil cation exchange, mining and resource extraction, and the presence of easily weathered minerals used in agriculture (lime) and urbanization (concrete). Increasing salts with strong bases and carbonates elevate acid neutralizing capacity and pH, and increasing sodium from salt pollution eventually displaces base cations on soil exchange sites, which further increases pH and alkalinization. Symptoms of the syndrome can include: infrastructure corrosion, contaminant mobilization, and variations in coastal ocean acidification caused by increasingly alkaline river inputs. Unless regulated and managed, the freshwater salinization syndrome can have significant impacts on ecosystem services such as safe drinking water, contaminant retention, and biodiversity.
We analyzed published rates of extracellular release (ER) of organic carbon to determine the primary constraints on this process and its importance to bacteria. From 16 studies we extracted observations of ER, particulate primary production (PP), and phytoplankton biomass. ln a regression model based on 225 observations, PP explained 69% of the variance in ER. From this model we estimate the average percent extracellular release (PER) to be 13% of total fixation. The slope of this relationship does not support the hypothesis that the PER declines with increasing productivity. Differences exist between marine and freshwater systems. In lakes, ER increases nonlinearly with productivity, resulting in very low PER in very eutrophic systems. In coastal marine and estuarine systems, ER increases linearly with productivity and the PER does not vary systematically. ER is not primarily related to phytoplankton biomass as predicted by passive diffusion models. Instead, ER appears to be constrained by the total availability of photosynthates. By comparing our model to an existing model of bacterial production and assuming a 50% growth efficiency, we estimate that ER amounts to less than half the C required for bacterial growth in most pelagic systems.
We examined the fate of planktonic bacterial production and the balance between bacterial growth and grazing mortality in the surface waters of Upton Lake, New York. Growth rates were measured by the incorporation of [3H]thymidine into DNA. Grazing rates on bacteria were determined with small cells produced by a mutant strain of Escherichia coli and made either fluorescent or radioactive to monitor feeding. Bacterial community turnover times calculated from either growth or grazing rates ranged from 1.5 to 16 d. On the basis of these data and results from 29 other studies, most bacterial communities appear to have turnover times substantially >1 d. Our measurements of feeding rates on bacteria frequently exceeded estimates of growth. Limitations of precision and doubts about the accuracy of methods make attempts to balance measurements of bacterial growth and grazing with current techniques unrealistic. The fate of bacterial production depends on planktonic community structure. Flagellates were the primary consumers of bacteria in winter and fall. At other times, Daphnia galeata consumed most of the bacterial production. Ciliates and rotifers were never important bacterial grazers. In Upton Lake large populations of Daphnia effectively "break" the microbial loop and funnel bacterial production to higher consumers.
Water temperatures are increasing in many streams and rivers throughout the US. We analyzed historical records from 40 sites and found that 20 major streams and rivers have shown statistically significant, long‐term warming. Annual mean water temperatures increased by 0.009–0.077°C yr −1 , and rates of warming were most rapid in, but not confined to, urbanizing areas. Long‐term increases in stream water temperatures were typically correlated with increases in air temperatures. If stream temperatures were to continue to increase at current rates, due to global warming and urbanization, this could have important effects on eutrophication, ecosystem processes such as biological productivity and stream metabolism, contaminant toxicity, and loss of aquatic biodiversity.
Cross-ecosystem subsidies to food webs can alter metabolic balances in the receiving (subsidized) system and free the food web, or particular consumers, from the energetic constraints of local primary production. Although cross-ecosystem subsidies between terrestrial and aquatic systems have been well recognized for benthic organisms in streams, rivers, and the littoral zones of lakes, terrestrial subsidies to pelagic consumers are more difficult to demonstrate and remain controversial. Here, we adopt a unique approach by using stable isotopes of H, C, and N to estimate terrestrial support to zooplankton in two contrasting lakes. Zooplankton (Holopedium, Daphnia, and Leptodiaptomus) are comprised of ≈ 20-40% of organic material of terrestrial origin. These estimates are as high as, or higher than, prior measures obtained by experimentally manipulating the inorganic (13)C content of these lakes to augment the small, natural contrast in (13)C between terrestrial and algal photosynthesis. Our study gives credence to a growing literature, which we review here, suggesting that significant terrestrial support of pelagic crustaceans (zooplankton) is widespread.
Abstract Phytoplankton blooms often follow nutrient enrichment. Differences among lakes in light‐absorbing dissolved organic carbon (DOC) may shift bloom thresholds to higher nutrient loads and thereby increase resilience of lakes to enrichment. To explore this idea, we measured resilience to experimental enrichment of two lakes with contrasting DOC concentrations. We compared bloom thresholds in both lakes using a model of phytoplankton response to DOC and nutrients, a dynamic time series indicator of resilience, and two empirical measures of stochastic resilience, mean exit time and median survival time. For the dynamic indicator and ecosystem model the lake with higher DOC was more resilient to enrichment. However, the distributions overlapped for stochastic indicators of resilience of the two lakes. These analyses show that DOC interacts with mixing depth and zooplankton biomass to affect resilience. Strong contrasts in DOC and many observations are needed to discern effects of DOC on resilience to enrichment.
