What is an example provided in the biotic factor?

Local Biotic Factors

Biotic factors can have positive or negative effects on algae in rivers. We know relatively little about commensalistic and mutualistic interactions, but some examples do exist. One example of commensalism would be the attachment of some benthic diatoms on stalks of other diatoms, which provides an advantage to species that can attach to stalks and has little negative effect on the stalked diatoms. Diatoms with endosymbiotic cyanobacteria provide an example of mutualistic interactions. Of the negative interactions, much more is known about the herbivory and competition versus allellopathic interactions and disease-like effects of fungi, bacteria, and viruses. Many have hypothesized that the latter two biotic stressors should have great effects in dense microbial assemblages like benthic algae. They are known to be important in lake and ocean phytoplankton. Unusual white circles in periphyton with high numbers of bacteria and fungi indicate ‘disease,’ but little investigation has pursued this line of research.

Competition is probably a more important determinant of algal biomass, function, and species composition for benthic algae than for phytoplankton. Phytoplankton seldom accumulate to sufficiently high densities to deplete nutrient concentrations in rivers because of the relatively short residence times of these organisms in their habitat. However, competition may be important in very slow flowing, lake-like rivers where residence time is sufficiently high to deplete nutrients or light by biological uptake or shading. These processes are thought to be important in benthic algal communities. If species membership is constrained to diatom-dominated communities, peak biomass of communities may be constrained by light and nutrient depletion. We know that light and nutrient availability within benthic algal mats decreases with increasing density; less light penetration and decreasing nutrient transport rates and cell nutrient content has been documented as diatom biomass increases on substrata. Per capita rates of metabolism and reproduction decrease with increasing benthic algal biomass. Species composition changes with increasing biomass of diatoms on substrata. All indicate autogenic changes in environment that are consistent with strong competitive regulation of benthic algal communities.

Herbivory is also an important determinant of algae in rivers. It is more frequently important for benthic algae than for phytoplankton because of the lack of time for zooplankton to accumulate in the water column of rivers. Low disturbance frequency by floods and drought is important for determining whether herbivory is important for benthic algae, too. When river conditions allow herbivores to accumulate to high densities, they can regulate biomass, function, and species composition of benthic algae in rivers. The importance of zebra mussels in some rivers is a good example of herbivores affecting river phytoplankton, but examples of zooplankton regulation are not common. Filter feeding invertebrates like blackflies and net-spinning caddisflies may also be important regulators of suspended algal abundance in streams, but this is not well understood.

Aquatic insects, snails, and some fish consume benthic algae, but aquatic insects and snails are the most important in most situations. Protozoa have also been shown to consume benthic algae, but their importance does not seem to be as great as cased caddisflies, mayflies, and snails that graze algae from substrata. These invertebrates can constrain diatom biomass to very low levels. Many of them can consume filamentous green algae during early stages of growth, but not after they have exceeded the size that can be controlled. They seem to avoid consumption of filamentous Cyanobacteria, but push it back from actively grazed areas or may knock it off the substrate. Invertebrate herbivores can reduce algal biomasses from 10 to 0.5 μg chl a cm−2. In addition, they selectively graze overstory versus understory diatoms (stalked and filamentous forms versus tightly adnate and prostrate forms). Although grazers consume algae, not all are killed. Estimates of algae passed alive and viable through guts of aquatic invertebrates often exceed 50%. In a sense, grazing of cells removes cells from the substratum and either causes death or emigration of the cells downstream when they are egested alive. Because of the importance of grazing as a process of removing algae from a location and conceptually within food webs, I have included it as one of the five fundamental processes.

Bioturbation is another process that affects benthic algal biomass. Invertebrates, fish, and terrestrial animals are common sources of disturbance of benthic algae as they move through streams. Diel patterns in algal drift are observed in some stream that correspond to the dawn and dusk activity patterns of aquatic invertebrates. Paths of disturbed benthic algae in shallow riffles can be observed in deeper upstream–downstream channels where fish have moved from pool to pool. Movement of fish in pools disturbs the development of periphyton and clouds the water. Raccoons and larger animals, such as manatees, crocodiles, hippopotamus, and humans, probably have great effects on benthic algae when moving, but these effects have not been quantified.

2.1 Abiotic-Biotic Relationships

Biotic factors change during a heat-initiated effect. Abiotic mechanisms, for example, hydrolysis and redox, combine with and influence biotic response, for example, photosynthesis and respiration. Notably, the growth and metabolism of the bacteria results in even more rapidly decreasing DO levels. Algae both consume DO for metabolism and produce DO by photosynthesis. The increase in temperature increases their aqueous solubility and the decrease in DO is accompanied by redox changes, for example, formation of reduced metal species, such as metal sulfides. This is also being mediated by the bacteria. The decrease in DO changes the redox potential of the water and sediment, initiating or increasing the rate of the reduction of metals. However, the opposite is true in the more oxidized regions, that is, where the metals are forming oxides. The increase in the metal compounds combined with the reduced DO, exacerbated by the increased temperatures, can act synergistically to make the conditions toxic for higher animals, for example, a fish kill [11]. This is an example of how an outcome can become a new stressor.

The first-order abiotic effect, that is, increased temperature, results in an increased microbial population. However, the growth and metabolism of the bacteria results in decreasing the DO levels, but the growth of the algae both consume DO for metabolism and produce DO by photosynthesis. Meanwhile a combined abiotic and biotic response occurs with the metals. The increase in temperature increases their aqueous solubility and the decrease in DO is accompanied by redox changes, for example, formation of reduced metal species, such as metal sulfides. This is also being mediated by the bacteria, some of which will begin reducing the metals as the oxygen levels drop (reduced conditions in the water and sediment). However, the opposite is true in the more oxidized regions, that is, the metals are forming oxides. The increase in the metal compounds combined with the reduced DO, combined with the increased temperatures can act synergistically to make the conditions toxic for higher animals (see Fig. 20.4). As Fig. 20.5 demonstrates, the movement of mass and energy downstream, both figuratively and literally, results in an adverse aquatic outcome, for example, a fish kill [11]. Predicting the likelihood of a fish kill can be quite complicated, with many factors that either mitigate or exacerbate the outcome. This is a classic depiction of Bayesian thinking, that is, prior events must update predicted outcomes, for example, posterior probabilities [12].