In aquatic ecosystems, most nutrients regenerate in the sediments

Because most cycling of elements takes place in an aqueous medium, the chemical and biochemical processes involved are similar in terrestrial and aquatic ecosystems. However, the location of decomposition differs between terrestrial and aquatic ecosystems. In terrestrial ecosystems, nutrients regenerate close to the location where they are taken up by producers. In aquatic ecosystems, most nutrients regenerate in sediments, which are often far below the surface waters that contain dominant producers such as phytoplankton. In addition, terrestrial ecosystems commonly experience aerobic decomposition whereas the sediments and deep waters of aquatic ecosystems commonly experience anaerobic decomposition, which is considerably slower. In this section we will investigate how decomposition operates in streams and wetlands, which receive much of their energy from leaves that blow in from the surrounding terrestrial environment. We will examine the important role of sedimentation in rivers, lakes, and oceans and explore how stratification of water affects the movement of regenerated nutrients in lakes and oceans.

Allochthonous Inputs to Streams and Wetlands

As we have discussed in previous chapters, streams and small forested wetlands receive a major portion of their energy from the surrounding terrestrial environment in the form of dead leaves that fall into the water. The decomposition process of leaves in a stream is similar to the process on land. As the leaves settle onto the bottom of the stream, the first stage is the leaching of soluble compounds followed by the shredding of the leaves into smaller pieces by invertebrates such as amphipods, isopods, and larval caddisflies. At the same time that leaves are being shredded, fungi and bacteria are working to decompose the leaves much as they do on land.

Given the similar processes in both the terrestrial and stream ecosystems, it is perhaps not surprising that the rate of leaf decomposition depends on the temperature of the water and the traits of the leaves. To determine the rate at which leaves decompose in streams, aquatic ecologists follow a protocol that is similar to the protocol used by terrestrial ecologists. They collect newly fallen leaves and place a weighed amount of dry leaves into a mesh bag that is submerged in the stream. The mesh bag allows aquatic invertebrates to enter the bag without losing any of the leaves. Over time, the bags are removed from the stream, dried, and reweighed to determine the amount of leaf mass that remains.

To determine how leaf traits affect leaf decomposition rates, researchers placed coarse-mesh bags of nine leaf species into a stream located in the Black Forest of Germany and removed the bags over time. The nine leaf species differed a great deal in their nitrogen, phosphorus, and lignin content. The researchers discovered that leaf decomposition rate was not related to the amount of nitrogen or phosphorus in the leaves but it was strongly associated with the lignin content of the leaves, as shown in Figure 21.16a.

Figure 21.16 Leaf decomposition in a stream. (a) When nine species of leaves in mesh bags were added to a stream in Germany, the decomposition rate was slower in leaf species that contained more lignin in their tissues. (b) The researchers also assessed the contribution of aquatic invertebrates to leaf decomposition by using large-mesh and fine-mesh leaf bags. After 55 days, the leaves in the fine-mesh bags, which excluded invertebrates, experienced 20 percent less decomposition compared to the large-mesh bags, which allowed invertebrates to enter. Error bars are standard errors.

Data from M. H. Schindler and M. O. Gessner, Functional leaf traits and biodiversity effects on litter decomposition in a stream, Ecology 90 (2009): 1641–1649.

The researchers also were interested in knowing how important the invertebrates were in the decomposition process. To answer this question, they placed a second set of leaves into fine-mesh leaf bags that prevented invertebrates from entering. As you can see in Figure 21.16b, leaf decomposition was about 20 percent higher in bags where invertebrates had been allowed to enter.

The small wetlands that exist in forests also receive a large proportion of their energy from leaves. Similar to leaves in streams, the decomposition of leaves in forested wetlands is largely tied to the lignin content. Moreover, the breakdown rate of the leaves has widespread effects on the entire food web and functioning of the ecosystem.

Decomposition and Sedimentation in Rivers, Lakes, and Oceans

In most rivers, lakes, and oceans, organic matter sinks to the bottom and accumulates in deep layers of sediments. While some nutrients are recycled in the surface waters when animals excrete waste or when microbes in the surface water decompose organic matter, most organic matter sinks to the sediments. As a result, most nutrients must come from the sediments, although they will return slowly to the productive surface waters.

The process of nutrient regeneration in sediments of aquatic ecosystems helps us understand many patterns of ecosystem productivity. For example, you might recall from Chapter 20 that the least productive aquatic ecosystems are the deep oceans where the benthos is far from the surface waters (see Figure 20.6). Shallow oceans are more productive in part because the sediments are much closer to the surface waters and can therefore regenerate nutrients more quickly through decomposition. The upwelling of water from the deep sediments to the surface brings nutrients from the site of regeneration to the site of algal productivity. In addition, some of the most productive regions of the ocean occur at the upwelling of water along the coasts of continents where currents draw deep, nutrient-rich water up to the surface (see Figure 5.11).

Stratification of Lakes and Oceans

The stratification of water, a phenomenon that we discussed in Chapter 6, further affects the availability of nutrients in aquatic ecosystems. The vertical mixing of water from the sediments to the surface can be hindered whenever surface waters have a different temperature and therefore a different density than that found in deep waters. The vertical mixing can affect primary production in two opposing ways. On the one hand, mixing can bring the deep, nutrient-rich water to the surface where phytoplankton can use it. On the other hand, mixing can carry phytoplankton down to the deep water, where they cannot sustain themselves because of the low light conditions. Under such conditions, primary production may shut down altogether in the deep water and little primary production will occur in the nutrient-rich waters at the surface.

Stratification happens in temperate and tropical lakes when the surface waters are warmed by the summer sunlight while the deeper waters stay cold and dense. Such stratification does not happen in polar lakes because their surfaces never become warm enough. In estuaries and oceans, stratification of the water happens when an input of less dense freshwater from rivers or melting glaciers is positioned above a layer of the more dense ocean saltwater.

Occasionally, ecosystems that stratify experience periods of vertical mixing. For example, temperate lakes in the spring and fall experience changes in the temperature of surface waters that eventually match the temperature of the deeper waters. When the temperatures become equal, spring and fall winds that blow along the surface of the lakes cause the entire lake to mix. Mixing can also happen in oceans. In areas where the surface water is less salty than the deep water, sunlight can slowly cause the surface waters to evaporate and leave the salt behind. At some point, the surface water becomes saltier than the deeper water and the surface water sinks, thereby causing the ocean water to circulate.

In this chapter, we have learned that elements cycle within and between ecosystems and that this cycling determines the availability of elements to organisms. We have also seen that human activities commonly affect element cycles in ways that are harmful to ecosystem function. Finally, we have seen how elements regenerate through weathering and the decomposition of organic matter. In “Ecology Today: Connecting the Concepts” we will apply these concepts to understand how logging and global change alters the functioning of an entire forest.

ECOLOGY TODAY CONNECTING THE CONCEPTS

CYCLING NUTRIENTS IN NEW HAMPSHIRE

Monitoring nutrient flows from a watershed. At the Hubbard Brook Experimental Forest in New Hampshire, the soils lie on bedrock that prevents water from percolating down, so all nutrients leached from the soils find their way into the streams that drain the watershed. At the bottom of the watershed, researchers constructed a catchment device to monitor water volumes and nutrient flow from the watershed.

Photo by U.S. Forest Service, Northern Research Station.

As we have seen throughout this chapter, producers play a key role in assimilating nutrients from the environment and holding the nutrients in their tissues to be passed on to consumers and decomposers. However, assessing the degree to which plants affect nutrient cycling in an ecosystem is a daunting task, in part because ecosystems are large and complex. Fifty years ago, researchers at the Hubbard Brook Experimental Forest in New Hampshire took on this challenge by selecting a number of forested watersheds and logging some of them. Then they monitored the changes in how nutrients moved in each watershed.

The Hubbard Brook Experimental Forest was an ideal location for this grand experiment. A layer of impenetrable rock under the soil prevents water from percolating into the deep groundwater. Because of this, all of the precipitation falling on the watershed is either taken up by plants or passes over and through the soil, ultimately ending up in the stream that leaves the watershed. Therefore, all water and nutrients not held by the plants or the soil exit the watershed through the stream, which allows the researchers to monitor the stream to measure the amount of water and nutrients leaving the ecosystem.

In 1962, a massive experiment was initiated in which researchers removed all of the trees from an entire watershed and sprayed it with herbicides for several years to suppress plant growth. As a control, adjacent watersheds were not logged. Since that time, ecologists have tracked how the ecosystem has responded to this disturbance.

Without plants to take up water and nutrients, the movement of elements in the ecosystem with suppressed plant growth changed dramatically. For example, the amount of water leaving the watershed in the stream increased severalfold. In addition, because the nitrates available in the soil were no longer being used by plants, there was a large increase in the amount of nitrates that leached out of the soil and into the water of the stream. In forests that were not logged, there was a net gain of soil nitrogen over time at a rate of 1 to 3 kg per ha; this increase came from precipitation and nitrogen fixation. However, in the logged watershed, there was a net loss of nitrogen at a rate of 54 kg per ha. Because nitrates do not bind well to soil, they leached out of the soil and into the stream after logging.

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Logging a watershed. To determine the role of plants in the cycling of nutrients, researchers at the Hubbard Brook Experimental Forest logged an entire watershed and then applied herbicides for several years to prevent plants from growing. Other watersheds were logged and allowed to regrow while still others were left unmanipulated as controls.

Photo by U.S. Forest Service, Northern Research Station.

Many other nutrients were also affected by the logging. For example, the researchers tracked the movement of calcium ions that come from precipitation and weathering of the bedrock. They found the vast majority of calcium ions came from the decomposition of detritus whereas less than 10 percent of the calcium came from weathering. When the region experienced years of acid precipitation, the inputs of calcium from precipitation and weathering could not keep pace with the calcium that leached from the soil. As a result, the forest experienced a net loss of calcium.

Today the Hubbard Brook Experimental Forest continues to provide insights into nutrient cycling. For example, in 2012 researchers reported that over a 5-decade period, the amount of nitrates leaving forested watersheds through the streams each year declined by more than 90 percent. They hypothesized that these declines could be due to changes in the species composition of the trees, climate change, or a recovery from past events that removed many of the trees. When they examined these alternatives, they discovered that changes in the composition of tree species only contributed to a small part of the decline in nitrates leaving the watershed. The forest today has fewer sugar maple trees, which have leaves that decompose and release nitrates rapidly, and more beech trees, which have leaves that decompose and release nitrates slowly. The researchers determined that climate change was responsible for about 40 percent of the reduction in nitrates in the stream; warmer temperatures in the late fall and early spring in recent years give plants a longer time to take up the nitrates, so fewer nitrates leach out of the soil and into the stream.

The researchers were surprised to find that 50 to 60 percent of the reduction in nitrates over the 5 decades was due to a recovery from historic disturbances, which they defined as events that removed more than 20 percent of the trees. At Hubbard Brook, these events include logging that took place in 1906 and 1917, a hurricane in 1938, and an ice storm in 1998. Just as we saw with the clear-cutting of a watershed in 1962, any removal of trees causes a short-term flush of nitrates out of the soils and into the stream. As the forests begin to come back, however, the soil recovers and holds on to more of the available nitrates. As the soil accumulates nitrates, the amount of nitrates leached into the stream declines.

The long-term experiment at Hubbard Brook shows us how natural events and human activities can dramatically alter the movement of elements within and between ecosystems in ways that can have major consequences for terrestrial and aquatic ecosystems.

SOURCES: Likens, G. E. 2004. Some perspectives on long-term biogeochemical research from the Hubbard Brook Ecosystem Study. Ecology 85: 2355–2362.

Bernal, S., et al. 2012. Complex response of the forest nitrogen cycle to climate change. PNAS 109: 3406–3411.