Aquatic organisms can be classified into four major groups, each varying in their biological characteristics, habitat, and adaptations, but linked within a complex network of ecological roles and relationships.
All these organisms are found in fresh water ecosystems, and an overview of each group may be understood by examining the following sub divisions:
1. Aquatic Organisms: Microorganisms
Microorganisms include members of the plant kingdom, protozoa, bacteria, and fungi. These organisms differ radically, and share only their small size; most are not visible without a microscope, though colonies of some can be seen with the naked eye.
They are present in large quantities everywhere and can survive extreme physical and chemical conditions.
Many microorganisms play foundational roles in aquatic ecosystems, capturing the sun’s energy through photosynthesis and, through their role in decomposition, releasing nutrients stored in organic tissue.
A further look at these organisms may be done by highlighting the different types;
Some of the smallest and most ancient organisms on earth, bacteria are present in virtually every environment and are abundant in all aquatic systems.
In rivers and streams, many of the bacteria wash in from the surrounding land, and their abundance can increase dramatically after a rainfall.
The abundance of bacteria is typically in the millions per millilitre (mL), and in the hundreds of millions per millilitre in especially productive or polluted waters.
If conditions are right, bacteria reproduce extremely rapidly by simple division to produce very large numbers in a short period of time.
Bacteria can be found suspended in the water, associated with decaying material (such as dead wood or leaves), or coating the surface of rocks, stones and sand grains as part of the biofilm (the slippery coating on hard surfaces in rivers). They can make up a large fraction of the living material in aquatic ecosystems.
Bacteria display the greatest range in the metabolic ability of any group of organisms. There are both autotrophic and heterotrophic bacteria. Heterotrophic bacteria are a crucial link in the decomposition of organic matter and the cycling of nutrients in aquatic systems.
Autotrophic bacteria are primary producers in aquatic systems as are true algae. For this reason, autotrophic bacteria (predominantly cyanobacteria) are often categorized as ‘algae’, though the organisms are by no means closely related.
Cyanobacteria used to be mistakenly called ‘blue-green algae’. Ecologically, much of what applies to algae is relevant to autotrophic bacteria.
These occur as single cells, and in filaments called hyphae.
Most aquatic fungi are microscopic; those known as hyphomycetes are the most abundant and important.
Fungi are heterotrophic, and, like heterotrophic bacteria, obtain their nutrition by secreting exoenzymes into their immediate environment, which break compounds down into simpler substances such that the fungi can easily absorb.
Fungi are critical to the decomposition of plant matter in aquatic ecosystems, because they are among the few organisms that can break down certain plant structural compounds such as cellulose and lignin.
These are microscopic, single-celled organisms that sometimes group together into colonies. There are both autotrophic and heterotrophic types of protozoa.
Unlike bacteria and fungi, which absorb dissolved organic compounds from their environment, heterotrophic protozoa (such as the amoebas and Paramecium) consume other organisms such as algae, bacteria, or other protists.
Together with other microorganisms, protozoa make up the biofilm coating sediments and hard surfaces on riverbeds, though some protozoa are free-swimming. Certain protozoa are parasites and cause diseases such as giardia (beaver fever).
iv. Algae and Phytoplankton
Several groups of largely autotrophic protists are referred to as algae. Like the term ‘microorganisms’ it is an informal term, used for convenience to describe microorganisms that carry out photosynthesis; the cyanobacteria are often included as algae.
Algae vary in size from microscopic to large colonies that can be macrophytes. Several types of algae – including phytoplankton – play an important role in supplying the energy at the base of many aquatic food webs.
Phytoplankton are small, microscopic plants that live suspended in the open water. They are generally more abundant in lakes than rivers, and are absent from fast-flowing streams, or where the rate at which the plants are washed downstream is greater than the rate at which they reproduce.
Damming a river leads to still-water conditions more suitable for phytoplankton, and nuisance algal blooms may develop in reservoirs. Inputs of nutrients, including nitrogen and phosphorus, can also lead to algal blooms.
Phytoplankton can exist as single cells or in chains or colonies. They are direct food sources for many zooplankton and some fish, and constitute the base of the food web in deep waters. Phytoplankton vary in their requirements for nutrients, light, and other conditions.
Fresh water ecosystems support a complex mixture of phytoplankton that can change markedly with environmental conditions. In rivers containing significant amounts of phytoplankton, the concentration of algal cells (number per unit volume) is generally highest when flows are lowest, while elevated suspended sediment loads during high flows can lead to reduced light and photosynthesis.
Some phytoplankton can cause taste and odour problems in water, and anoxic conditions that can kill fish. Some cyanobacteria produce toxins lethal to various fish, wildlife, and domestic species.
v. Periphyton and Biofilm
Algae, bacteria, fungi, protozoa, and the breakdown products of dying cells form layers on submerged surfaces, including bottom sediment, rocks, submerged leaves and branches, and macrophytes.
The term periphyton refers to a layer consisting mainly of algae, but the entire assemblage of layers is often known as biofilm. Periphyton is an important food source in shallow, stony rivers with adequate light penetration.
Heterotrophic organisms, including larger invertebrates such as snails and insects, scrape the biofilm from surfaces, while some larger animals, such as fish, also feed on biofilm.
Biofilm can be important in absorbing or breaking down chemical contaminants as well. Seasonal changes in the abundance of periphyton reflect fluctuations in river discharge, as layers of algal cells build up in times of low or decreasing flow, and wash away during flood periods.
2. Aquatic Organisms: Plants
Macrophytes (literally ‘large plants’) are individual aquatic plants that can be seen by the naked eye, and can be categorized based on where and how they grow. Based on this categorization, the following types can be identified.
Rooted macrophytes: These are always rooted in the riverbed or lake substrate, and are thus restricted to areas where flow is low enough to permit fine sediments to accumulate.
Rooted macrophytes may have leaves entirely submerged (under the water), floating on the surface, or emergent above the surface.
In turbid water, little light penetrates and photosynthesis is restricted, hence only plants with floating or emergent leaves can thrive.
Rooted macrophytes may extract nutrients from the substrate as well as absorbing them from the water as algae do.
Floating aquatic macrophytes are rootless plants that persist only in backwater areas where the flow slackens otherwise they are carried downstream. Because their photosynthetic surfaces are above the water surface, these plants can grow in deep, turbid water and places where rooting sites are sparse.
Macrophyte abundance can fluctuate seasonally as a result of scouring of the bottom sediments and washout of plants during heavy rains. For this reason, the number of macrophytes in river channels generally peaks during periods of low flow.
Aquatic macrophytes are important in many aquatic ecosystems, especially wetlands, slower moving water in streams and rivers, and in shallower areas of lakes.
Aquatic macrophytes add three-dimensional complexity to aquatic habitat, and can provide habitat, refuge, and spawning areas for animals such as aquatic insects and fish, as well as a surface for periphyton growth.
As they are primary producers, aquatic macrophytes produce organic matter which can be eaten by some fish; however, most of this plant material is unpalatable to herbivores while it is alive.
Energy is transferred to animals primarily when the dead plant tissue and associated decomposers are eaten.
Large populations of aquatic macrophytes can have negative effects on aquatic ecosystems and the people that rely on them. In some cases, floating plants are so numerous that they form dense mats covering the water surface.
Their buoyant leaf crowns merge above the surface while the root masses dangle below into the water. The interlocking vegetation mat blocks light penetration down the water column and prevents the growth of other plants.
In extreme cases, the underlying water becomes deoxygenated, and floating plants become a nuisance by inhibiting the passage of boats and interfering with fishing.
Invasive species of macrophytes can be particularly disruptive to natural aquatic ecosystems.
ii. Riparian Vegetation
Riparian vegetation is plant growth that lines the banks of rivers and other inland water bodies. These plants protect river banks from wave action and erosion, and offer shelter, feeding, and breeding areas for fish, birds and other organisms.
Leaves, twigs, and other organic matter from riparian vegetation can provide significant quantities of organic matter to streams and rivers.
The riparian zone can contain a variety of plants from grasses to trees often in a gradual transition with distance from the bank, reflecting different species’ tolerances for soil saturation.