1052

Life, of course, is not limited to the land; diverse communities thrive in both freshwater and marine environments. Organisms that live in water face functional challenges distinct from those on land, and this helps to explain the nature of aquatic biomes. From the point of view of evolution, terrestrial organisms evolved from ancestors living in water, but to understand ecology, it is helpful to go from familiar terrestrial biomes to aquatic examples we generally experience only indirectly.

The most obvious distinction between terrestrial and aquatic biomes is that water is denser than air. For primary producers, buoyancy in water can provide mechanical support, reducing or eliminating the need for structural tissues such as wood. Also, primary producers on land are tethered to the ground both by gravity and the availability of nutrients and water, but this is not the case in aquatic biomes. In aquatic environments, nutrients are dissolved in encompassing waters, and by controlling their buoyancy, aquatic primary producers can live far from shore in surface waters bathed in sunlight.

Just as on land, the structure and distribution of primary producers governs patterns of consumer organisms. Aquatic plants and macroscopic algae are important primary producers in shallow aquatic environments where they can take advantage of nutrients transported from land while remaining anchored to the riverbed, lakebed, or seafloor. In larger lakes and the open ocean, however, phytoplankton—photosynthetic microorganisms that float in sunlit waters—are predominant primary producers, combining high rates of primary production with low biomass.

In shallow aquatic environments such as lake margins and coastal marine settings, much plant, algal, and animal debris accumulates in sediments, supporting luxuriant growth by bacteria. As on land, these decomposers provide food for many larger consumers, including the larvae and juveniles of commercially important fish and invertebrate species. Fungi, so important in terrestrial biomes as decomposers, are generally thought to play only a minor role in aquatic environments, although recent surveys of marine sediments show that where organic matter is abundant and oxygen is absent, microscopic fungi can be major components of the decomposer guild. Bacteria also play a critical role in cycling the remains of phytoplankton and other open-water organisms, both within the water and on deep lakebeds or seafloors.

Grazers such as snails and fish feed on marginal plants and algae, while single-celled protists and microscopic animals called zooplankton consume phytoplankton that floats in the water. As on land, secondary consumers such as other fish or squid feed on the grazers, adding complexity to food webs. Plankton, whether photosynthetic or heterotrophic, generally have only limited powers of locomotion and so drift with the current. In contrast, secondary consumers collectively called nekton—fish, squid, whales, and others—can move actively through their watery surroundings. Because oxygen gas dissolves in water only to a limited extent, surface waters directly beneath the atmosphere will have an O₂ concentration about 20 times lower than that of the air. This compels larger animals to be moving constantly to keep their gill systems well oxygenated, or to live in places such as cold running streams and rocky marine coastlines where currents naturally produce well-oxygenated water.

1053

1054

1055

1056

1057

The main freshwater biomes are lakes and rivers (Fig. 48.12). These vary tremendously in climate, and also in nutrient input and oxygen availability. In general, the turbulent flow of water ensures that rivers are well oxygenated. Some lakes, however, are physically stratified, meaning that the density of bottom waters keeps them from mixing upward with shallow waters. Organic remains that sink to deep waters provide food for respiring bacteria, and this draws down oxygen levels. Where nutrient influx is high, such lakes can become anoxic in their deeper parts; this is particularly common where human activities have increased nutrient loads entering the lake (Chapter 49).

Only about 2.5% of Earth’s water is fresh, and most of this water occurs in glaciers, permafrost, and groundwater within soils. Overwhelmingly, the water on our planet is seawater. Covering 71% of Earth’s surface and reaching a depth of nearly 11,000 m (almost 7 miles), the oceans constitute Earth’s largest biomes. In fact, the oceans are so large and so diverse in their biology that ecologists commonly subdivide them into distinct realms based on depth and proximity to the shoreline (Fig. 48.13).

The dimensions of the oceans matter because they determine the distribution of nutrients, oxygen, and light. Runoff from the continents (supplemented by nitrogen fixation by marine cyanobacteria) provides most of the nutrients required by primary producers in the sea, but because sunlight rarely penetrates much deeper than about 200 m (the photic zone), most of the ocean’s volume is off limits for photosynthetic organisms. Life in the deep sea is largely sustained by detritus that rains downward from surface waters, although chemoautotrophic bacteria and archaeons contribute as well. Particle- and filtering-feeding animals, as well as microbes, respire this rain of detritus, returning phosphorus and nitrogen to the water. These nutrients eventually return to the surface in upwelling waters, mostly along continental margins, where they support locally high rates of photosynthesis. Upwelling regions make up only about 1% of the ocean’s surface area, but by some estimates, they support more than 20% of all commercial fish populations.

Many different groups of algae contribute to primary production on continental shelves, with diatoms, coccolithophorids, and dinoflagellates dominating in most regions. Because these algae turn over rapidly—most are eaten by grazers or lysed by viruses within a few days—their total biomass is commonly lower than that of grazing zooplankton, inverting the trophic pyramid characteristic of terrestrial ecosystems (Chapter 47). In the middle of large ocean basins, broad areas called gyres that are bounded by major currents, lie far from continental and upwelling sources of nutrients; their low primary productivity is dominated by cyanobacteria and tiny algae discovered only recently.

1058

Viruses play a particularly large role in marine biomes, lysing up to 30% of phytoplankton daily in coastal ecosystems and releasing large amounts of organic molecules into the water column. This organic matter supports large populations of respiring bacteria, forming a microbial loop in the carbon cycle (Chapter 25). In turn, bacterial populations in the microbial loop support diverse populations of larger consumers.

Earlier in this chapter, we noted that winds drive ocean circulation, but winds affect only the uppermost waters of the sea. The surface layer that is mixed by the wind, broadly equivalent to the photic zone, is well oxygenated, but in deeper waters, oxygen can be depleted by respiration as microbes feed on sinking organic matter. In a few places, where surface primary production is high, the oxygen level in water masses just beneath the mixed layer declines to zero, forming an oxygen-minimum zone with distinctive microbial populations and only a limited presence of zooplankton and animals. On the deep seafloor and in the waters above it, oxygen is present, but food is sparse, consisting only of a slow rain of particles from shallower depths. Life persists in this biome as scattered populations of animals (especially fish, brittle stars, and polychaete worms), heterotrophic protists, and microbes. The exception occurs at hydrothermal vents, where high concentrations of sulfide, hydrogen, and methane support primary production by chemosynthetic bacteria and archaeons. These, in turn, support locally dense populations of animals.

1059

Key marine biomes are summarized in Fig. 48.14.