8.2: The structure of aquatic ecosystems

Abiotic factors in Aquatic Biomes

Like terrestrial biomes, the aquatic biome is influenced by abiotic factors. In the case of this biome, the abiotic factors include light, temperature, flow, and dissolved nutrients.The aquatic medium—water— has different physical and chemical properties than air. Even if the water in a pond or other body of water is perfectly clear (there are no suspended particles), water, on its own, absorbs light. As one descends deep enough into a body of water, eventually there will be a depth at which the sunlight cannot reach. While there are some abiotic and biotic factors in a terrestrial ecosystem that shade light (like fog, dust, or insect swarms), these are not usually permanent features of the environment. The importance of light in aquatic biomes is central to the communities of organisms found in both freshwater and marine systems because it controls productivity through photosynthesis.

Definition: Aquatic Biome

The aquatic biome is the largest of all the biomes, covering about 75 percent of Earth’s surface. This biome is usually divided into two categories: freshwater and marine. Typically, freshwater habitats are less than 1 percent salt. Marine life, however, has to be adapted to living in a habitat with a high concentration of salt. Freshwater habitats include ponds, lakes, rivers, and streams, while marine habitats include the ocean and salty seas.

In addition to light, solar radiation warms bodies of water and many exhibit distinct layers of water at differing temperatures. The water temperature affects the organisms’ rates of growth and the amount of dissolved oxygen available for respiration.

The movement of water (flow) is also important in many aquatic biomes. In rivers, the organisms must obviously be adapted to the constant movement of the water around them, but even in larger bodies of water such as the oceans, regular currents and tides impact availability of nutrients, food resources, and the presence of the water itself.

Finally, all natural water contains dissolved salts. Fresh water contains low levels of such dissolved substances because the water is rapidly recycled through evaporation and precipitation. The oceans have a relatively constant high salt content. Aquatic habitats at the interface of marine and freshwater ecosystems have complex and variable salt environments that range between freshwater and marine levels. These are known as brackish water environments. Lakes located in closed drainage basins concentrate salt in their waters and can have extremely high salt content that only a few and highly specialized species are able to inhabit.

Dissolved nutrients such as nitrogen, phosphorus, and silica, occur in low amounts but are important for microscopic aquatic plants known as phytoplankton. Phytoplankton serve as the basis of the food chain in most marine systems. Phytoplankton growth is often limited by the availability of these, and other nutrients, and thus systems with high nutrients support rapid growth of phytoplankton who in turn support diverse communities of aquatic organisms. Nutrients availability in freshwater systems is often high. In marine systems, nutrient availability is highly variable and is influenced by wind, ocean depth, proximity to shore, and circulation of deep ocean currents.

Freshwater Systems

The aquatic (freshwater, or partly freshwater) biome include lakes, ponds, and wetlands (standing water) as well as rivers and streams (flowing water). Freshwater biomes are found in terrestrial landscapes and are therefore connected with abiotic and biotic factors influencing these terrestrial biomes.

Lakes and Ponds

Lakes and ponds can range in area from a few square meters to thousands of square kilometers. A lake is an area filled with water, localized in a basin, surrounded by land, and distinct from any river or other outlet that serves to feed or drain the lake. Lakes are typically larger and deeper than ponds, which also lie on land, though there are no official or scientific definitions. Most lakes are fed and drained by rivers and streams.

Temperature is an important abiotic factor affecting living things found in lakes and ponds. During the summer in temperate regions, thermal stratification of deep lakes occurs when the upper layer of water is warmed by the Sun and does not mix with deeper, cooler water (Figure \(\PageIndex{1}\)). The process produces a sharp transition between the warm water above and cold water beneath and has a large influence on the animal and plant life inhabiting the lake. The two layers do not mix until cooling temperatures and winds break down the stratification and the water in the lake mixes from top to bottom.

During the period of stratification, most of the productivity occurs in the warm, well-illuminated, upper layer, while dead organisms slowly rain down into the cold, dark layer below where decomposing bacteria and cold-adapted species such as lake trout exist. Like the ocean, lakes and ponds have a photic layer in which photosynthesis can occur. Phytoplankton (algae and cyanobacteria) are found here and provide the base of the food web of lakes and ponds. Zooplankton, such as rotifers and small crustaceans, consume these phytoplankton. At the bottom of lakes and ponds, bacteria in the aphotic zone break down dead organisms that sink to the bottom.

Figure \(\PageIndex{1}\): The effect of seasons on mixing and stratification of large freshwater lakes. (CC-BY-SA 4.0: Dimictic lake by wikimedia Commons)

Rivers and Streams

Rivers and the narrower streams that feed into the rivers are continuously moving bodies of water that carry water from the source or headwater to the mouth at a lake or ocean. The largest rivers include the Nile River in Africa, the Amazon River in South America, and the Mississippi River in North America (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Rivers range from (a) narrow and shallow to (b) wide and slow moving. (modification of work by Cory Zanker; credit b: modification of work by David DeHetre)

Abiotic features of rivers and streams vary along the length of the river or stream. Streams begin at a point of origin referred to as source water. The source water is usually cold, low in nutrients, and clear. The channel (the width of the river or stream) is narrower here than at any other place along the length of the river or stream. Headwater streams are of necessity at a higher elevation than the mouth of the river and often originate in regions with steep grades leading to higher flow rates than lower elevation stretches of the river.

Headwater streams are often clear because they are closer to their origin and fast moving, resulting in minimal siltation. Photosynthesis here is mostly attributed to algae that are growing on rocks; the swift current inhibits the growth of phytoplankton. While photosynthesis in the water may be partially blocked by overhead tree branches and leaves, those same leaves add energy to the stream. When the leaves decompose, the organic material and nutrients in the leaves are returned to the water. The leaves also support a food chain of invertebrates that eat them and are in turn eaten by predatory invertebrates and fish. Plants and animals have adapted to this fast-moving water. For instance, some species of mayfly (phylum Arthropoda) have flattened bodies and legs with modified claws to help them cling to the underside of submerged rocks. This body form reduces drag and allows these species to benefit from the high oxygen concentrations in fast-moving currents without being dislodged. Freshwater trout species are an important predator in these fast-moving rivers and streams.

As the river or stream flows away from the source, the width of the channel gradually widens, the current slows, and the temperature characteristically increases. The increasing width results from the increased volume of water from more and more tributaries. Gradients are typically lower farther along the river, which accounts for the slowing flow. With increasing volume can come increased silt, and as the flow rate slows, the silt may settle, thus increasing the deposition of sediment. Phytoplankton can also be suspended in slow-moving water. Therefore, the water will not be as clear as it is near the source. The water is also warmer as a result of longer exposure to sunlight and the absence of tree cover over wider expanses between banks. Worms and insects can be found burrowing into the mud. Predatory vertebrates include waterfowl, frogs, and fishes. In heavily silt-laden rivers, these predators must find food in the murky waters, and, unlike the trout in the clear waters at the source, these vertebrates cannot use vision as their primary sense to find food. Instead, they are more likely to use taste or chemical cues to find prey.

When a river reaches the ocean or a large lake, the water typically slows dramatically and any silt in the river water will settle. Rivers with high silt content discharging into oceans with minimal currents and wave action will build deltas, low-elevation areas of sand and mud, as the silt settles onto the ocean bottom. Rivers with low silt content or in areas where ocean currents or wave action are high create estuarine areas where the freshwater and saltwater mix.

Wetlands

Wetlands are environments in which the soil is either permanently or periodically saturated with water. Wetlands are different from lakes and ponds because they are shallower. Because of this, they exhibit a near continuous cover of emergent vegetation. Emergent vegetation consists of wetland plants that are rooted in the soil but have portions of leaves, stems, and flowers extending above the water’s surface. There are several types of wetlands including marshes, swamps, bogs, mudflats, and salt marshes (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): Located in southern Florida, Everglades National Park is vast array of wetland environments, including sawgrass marshes, cypress swamps, and estuarine mangrove forests. Here, a great egret walks among cypress trees. (credit: NPS)

Freshwater marshes and swamps are characterized by slow and steady water flow. Bogs develop in depressions where water flow is low or nonexistent. Bogs usually occur in areas where there is a clay bottom with poor percolation. Percolation is the movement of water through the pores in the soil or rocks. The water found in a bog is stagnant and oxygen depleted because the oxygen that is used during the decomposition of organic matter is not replaced. As the oxygen in the water is depleted, decomposition slows. This leads to organic acids and other acids building up and lowering the pH of the water. At a lower pH, nitrogen becomes unavailable to plants. This creates a challenge for plants because nitrogen is an important limiting resource. Some types of bog plants (such as sundews, pitcher plants, and Venus flytraps) capture insects and extract the nitrogen from their bodies. Bogs have low net primary productivity because the water found in bogs has low levels of nitrogen and oxygen.

Estuaries

Estuaries are biomes that occur where a river, a source of freshwater, meets the ocean. Therefore, both freshwater and saltwater are found in the same vicinity; mixing results in a diluted (brackish) salt water. Estuaries form protected areas where many of the offspring of crustaceans, mollusks, and fish begin their lives. Salinity is an important factor that influences the organisms and the adaptations of the organisms found in estuaries. The salinity of estuaries varies and is based on the rate of flow of its freshwater sources. Once or twice a day, high tides bring salt water into the estuary. Low tides occurring at the same frequency reverse the current of salt water (Figure \(\PageIndex{4}\)).

Figure \(\PageIndex{4}\): As estuary is where fresh water and salt water meet, such as the mouth of the Klamath River in California. (credit: U.S. Army Corps of Engineers)

The daily mixing of fresh water and salt water is a physiological challenge for the plants and animals that inhabit estuaries. Many estuarine plant species are halophytes, plants that can tolerate salty conditions. Halophytic plants are adapted to deal with salt water spray and salt water on their roots. In some halophytes, filters in the roots remove the salt from the water that the plant absorbs. Animals, such as mussels and clams (phylum Mollusca), have developed behavioral adaptations that expend a lot of energy to function in this rapidly changing environment. When these animals are exposed to low salinity, they stop feeding, close their shells, and switch from aerobic respiration (in which they use gills) to anaerobic respiration (a process that does not require oxygen). When high tide returns to the estuary, the salinity and oxygen content of the water increases, and these animals open their shells, begin feeding, and return to aerobic respiration.

Marine Systems

The ocean is a continuous body of saltwater that is relatively uniform in chemical composition. It is a weak solution of mineral salts and decayed biological matter. The ocean is categorized by several zones (Figure \(\PageIndex{5}\)). All of the ocean’s open water is referred to as the pelagic realm (or zone). The benthic realm (or zone) extends along the ocean bottom from the shoreline to the deepest parts of the ocean floor. From the surface to the bottom or the limit to which photosynthesis occurs is the photic zone (approximately 200 m or 650 ft). At depths greater than 200 m, light cannot penetrate; thus, this is referred to as the aphotic zone. The ocean is, on average, 4267 m or 14,000 ft deep. Thus, the majority of the ocean is aphotic and lacks sufficient light for photosynthesis.

Figure \(\PageIndex{5}\): The ocean is divided into different zones based on water depth, distance from the shoreline, and light penetration. (CC BY; Open Stax)

Intertidal Zone

The intertidal zone (Figure \(\PageIndex{5}\)) is the oceanic region that is closest to land. With each tidal cycle, the intertidal zone alternates between being inundated with water and left high and dry. Generally, most people think of this portion of the ocean as a sandy beach. In some cases, the intertidal zone is indeed a sandy beach, but it can also be rocky, muddy, or dense with tangled roots in mangrove forests.

The intertidal zone is an extremely variable environment because of tides. Organisms may be exposed to air at low tide and are underwater during high tide. Therefore, living things that thrive in the intertidal zone are often adapted to being dry for long periods of time. The shore of the intertidal zone is also repeatedly struck by waves and the organisms found there are adapted to withstand damage from the pounding action of the waves (Figure \(\PageIndex{6}\)). The exoskeletons of shoreline crustaceans (such as the shore crab, Carcinus maenas) are tough and protect them from desiccation (drying out) and wave damage. Another consequence of the pounding waves is that few algae and plants establish themselves in constantly moving sand or mud. Thus in sandy or mud bottom intertidal communities many organisms burrow into the substrate.

Figure \(\PageIndex{6}\): Sea stars, sea urchins, and mussel shells are often found in the intertidal zone, shown here in Kachemak Bay, Alaska. (credit: NOAA)

Neritic Zone

The neritic zone is the relatively shallow part of the ocean above the drop-off of the continental shelf, approximately 200 meters (660 ft) in depth. From the point of view of marine biology it forms a relatively stable and well-illuminated environment for marine life, from plankton up to large fish and corals, while physical oceanography sees it as where the oceanic system interacts with the coast. When the water is relatively clear, photosynthesis can occur in the neritic zone. The water contains silt and is well-oxygenated, low in pressure, and stable in temperature. These factors all contribute to the neritic zone having the highest productivity and biodiversity of the ocean. Phytoplankton, including photosynthetic bacteria and larger species of algae, are responsible for the bulk of this primary productivity. Zooplankton, protists, small fishes, and shrimp feed on the producers and are the primary food source for most of the world’s fisheries. The majority of these fisheries exist within the neritic zone.

Coral Reefs

Coral reefs are ocean ridges formed by marine invertebrates living in warm shallow waters within the photic zone of the ocean. They are found within 30˚ north and south of the equator. Corals are adapted to warm clear waters. The Great Barrier Reef is a well-known, and largest reef system in the world, located several miles off the northeastern coast of Australia. Other coral reefs are fringing islands, which are directly adjacent to land, or atolls, which are circular reefs surrounding a former island that is now underwater.

The coral-forming colonies of animals (related to jelly fish and sea anemones) secrete a calcium carbonate skeleton. These calcium-rich skeletons slowly accumulate, thus forming the underwater reef (Figure \(\PageIndex{7}\). Corals found in shallower waters (at a depth of approximately 60 m or about 200 ft) have a mutualistic relationship with photosynthetic algae. The relationship provides corals with the majority of the nutrition and the energy they require. The waters in which these corals live are nutritionally poor and, without this mutualism, it would not be possible for large corals to grow because there are few planktonic organisms for them to feed on. Some corals living in deeper and colder water do not have a mutualistic relationship with algae; these corals must obtain their energy exclusively by feeding on plankton using stinging cells on their tentacles.

Coral reefs are one of the most diverse biomes. It is estimated that more than 4000 fish species inhabit coral reefs. These fishes can feed on coral, the cryptofauna (invertebrates found within the calcium carbonate structures of the coral reefs), or the seaweed and algae that are associated with the coral. These species include predators, herbivores, or planktivores. Predators are animal species that hunt and are carnivores or “flesh eaters.” Herbivores eat plant material, and planktivores eat plankton.

Figure \(\PageIndex{7}\): Coral reefs are formed by the calcium carbonate skeletons of coral organisms, which are marine invertebrates in the phylum Cnidaria. (credit: Terry Hughes)

Kelp Forests

Kelp forests are underwater areas with a high density of kelp, which covers a large part of the world's coastlines. They are recognized as one of the most productive and dynamic ecosystems on Earth (Figure \(\PageIndex{8}\)). Although algal kelp forest combined with coral reefs only cover 0.1% of Earth's total surface, they account for 0.9% of global primary productivity. Kelp forests occur worldwide throughout temperate and polar coastal oceans.

Figure \(\PageIndex{8}\): The kelp (a type of brown algae) of a kelp forest creates 3-dimensional structure and habitat for a diversity of fish, invertebrates, and mammals. (public domatin; Sara Kappus taken on Santa Catalina Island)

Kelp forests are mostly found in cool, shallow, nutrient-rich water near coasts. Kelp is a brown alga, which requires access to light in order to photosynthesize - this is the reason for their abundance in shallow coastal water, Kelp forests provide a unique habitat for marine organisms and are a source for understanding many ecological processes. Over the last century, they have been the focus of extensive research, particularly in trophic ecology, and continue to provoke important ideas that are relevant beyond this unique ecosystem. For example, kelp forests can influence coastal oceanographic patterns and provide many ecosystem services

Kelp forests often draw comparisons to terrestrial forests, however the two ecosystems are distinct in that kelp is an algae - not a member of the Plant Kingdom. The structure of all kelp bears three universal morphological characteristics: holdfasts, stipes, and fronds. The holdfasts, which serve as an anchor for kelp, must be attached to hard substrates such as rocks or sand. Although they resemble roots on terrestrial plants, holdfasts do not transport water or nutrients through their stipe. The stipe, analogous to the stem, gives structural support to the algae. Fronds are blades extending from the stipe. The blades serve as the photosynthetic and nutrient uptake center for the organism.

Kelp forests are an important foundation species, which provide large areas of habitat within the ocean. Kelp forests have a wide variety of inhabitants within their boundaries: invertebrates frequently graze on the blades, fish find shelter in the fronds, and invertebrates - such as brittle stars, sea stars, anemones, sponges and tunicates - live in the holdfast. Kelp is also the single largest source of fixed carbon within their ecosystems. Pieces of decomposing kelp sink deeper into the ocean, or wash up on beaches, providing nutrients organisms in other ecosystems. Thus, kelp derived particulate organic matter (POM) is an important food source for fauna both within and outside the kelp forest.

Since kelp is capable of attenuating waves and dampening their energy, many animals are able to seek refuge in the forests. Various species of kelp tolerate ocean storms differently. In fact, along the California coast giant kelp has been known to dominate in years with less turbulent sea conditions, while bull kelp is more dominant in open waters and years of turbulent conditions.

Oceanic zone

Photic Zone

Beyond the neritic zone is the open ocean area known as the oceanic zone, which is further divided into 3 layers based on sunlight availability (the photic zone, aphotic zone and abyssal zone) (Figure \(\PageIndex{5}\)). Within the Photic zone there is sufficient light for net photosynthesis to occur. Abundant phytoplankton and zooplankton support populations of fish, sea birds, and marine mammals. Nutrients are scarce and this is a relatively less productive part of the marine biome when compared to the Neritic zone. When photosynthetic organisms and the organisms that feed on them die, their bodies fall to the bottom of the ocean and provide food for deep ocean scavengers.

Aphotic Zone

The depth at which less than one percent of sunlight reaches begins the aphotic zone. While most of the ocean’s biomass lives in the photic zone, the majority of the ocean’s water lies in the aphotic zone. Bioluminescence is more abundant than sunlight in this zone.

Figure \(\PageIndex{9}\): The deep-sea pandalid shrimp Heterocarpus ensifer and a photo of the same animal ‘vomiting’ light from glands located near its mouth.Photographed during the Bioluminescence and Vision 2015 expedition, as scientists sought to gain a better understanding of how and why organisms create their own light. (CC BY-SA; NOAA Ocean Explorer via Wikipedia Commons)

Abyssal zone

The deepest part of the ocean is the abyssal zone, which is at depths of 4000 m or greater. The abyssal zone (Figure \(\PageIndex{5}\)) is very cold and has very high pressure, very low or no oxygen content, and high nutrient content as the dead and decomposing material that drifts down from the layers above. There are a variety of invertebrates and fishes found in this zone, but the abyssal zone does not have photosynthetic organisms. Chemosynthetic bacteria use the hydrogen sulfide and other minerals emitted from deep hydrothermal vents. These chemosynthetic bacteria use the hydrogen sulfide as an energy source and serve as the base of the food chain found around the vents.

Benthic Realm

Beneath the oceanic zone is the benthic realm, the deepwater region beyond the continental shelf (Figure \(\PageIndex{5}\)). The bottom of the benthic realm is composed of sand, silt, and dead organisms. Temperature decreases as water depth increases. This is a nutrient-rich portion of the ocean because of the dead organisms that fall from the upper layers of the ocean. Because of this high level of nutrients, a diversity of fungi, sponges, sea anemones, marine worms, sea stars, fishes, and bacteria exists.

Attribution:

This page is a modified derivative of:

• 20.4 Aquatic and Marine Biomes by OpenStax CC BY
• Lake by Wikipedia, license CC-BY-SA 3.0
• Neritic Zone by Wikipedia, license CC-BY-SA 3.0
• Aphotic Zone Wikipedia, license CC-BY-SA 3.0
• Kelp Forests by Wikipedia, license CC-BY-SA 3.0
• Kelp Forest Ecosystems by LibraText, license CC BY-NC-SA 4.0