Food Webs

Studies of salt marshes have made important advances in food web theory. Early papers focused on primary productivity measurements and attempts to explain differences in rates within and among salt marshes. The energy-subsidy model described S. alternifloras high productivity at low elevations as a function of increased rates of nutrient delivery and waste removal, due to frequent tidal inundation. It also explained how salt marshes with decreasing tidal energy across Long Island, New York, had a corresponding decrease in S. alterniflora productivity. R. E. Turner added the role of climate by relating higher productivity of S. alterniflora to warmer latitudes.

In the 1960s, E. Odum's interest in energy flow led several investigators at the University of Georgia to quantify productivity, consumption, and decomposition of various components of Sapelo Island salt marshes. J. Teal's energy-flow diagram depicted Georgia's S. alterniflora marsh as exporting organic matter. Although estimated by subtraction rather than measurement, detrital export became a textbook example of how ecosystems channel and dissipate energy.

Later, advances were made in exploring the quantity and fate of detritus derived from salt marsh primary producers. J. Teal's suggestion that substantial organic matter is transported to estuarine waters supported E. Odum's 'outwelling hypothesis', that estuarine-derived foods drive coastal food webs and benefit commercial fisheries. A number of ecosystem-scale tests of outwelling ensued, and although outwelling did not prove to be universal, the research demonstrated connectivity between riverine, salt marsh, and open-water ecosystems. Also, the copious detrital organic matter provided by salt marshes was shown to be high in nutritional value once detrital particles were enriched by microorganisms, but microalgae were also shown to be an important food source. Even though their standing crop is low, high turnover rates lead to high primary productivity. In salt marshes with ample light penetration through the vascular plant canopy, microalgae can be as productive as macrophytes, and some species (notably cyanobacteria) are much richer in proteins and lipids. Algae also hold much of the labile nitrogen in salt marshes, widely thought to be the limiting factor for growth of invertebrate grazers.

Food webs are driven by both 'bottom-up' or 'top-down' processes. Evidence for bottom-up control of trophic interactions comes from experimental addition of nitrogen. Nitrogen has been shown to limit algae, vascular plants, grazers, and predatory invertebrates in nearly every salt marsh field experiment. Recently, however, P. V. Sundareshwar and colleagues showed that phosphorus can limit microbial communities in coastal salt marshes.

Despite widespread evidence for bottom-up effects, there is expanded recognition of the top-down role of consumers in regulating salt marsh food webs. Populations of lesser snow geese have increased due to agricultural grains that are left in the fields after harvest, and large flocks now cause large-scale destruction of vegetation in Arctic salt marshes due to rampant herbivory. In Atlantic salt marshes of southern USA, snail herbivory accompanies drought-induced die-back of S. alterniflora.

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