The function of marine critical transition zones and the importance of sediment biodiversity

Estuaries and coastal wetlands are critical transition zones (CTZs) that li nk land, freshwater habitats, and the sea. CTZs provide essential ecologica l functions, including decomposition, nutrient cycling, and nutrient produc tion, as well as regulation of fluxes of nutrients, water, particles, and o rganisms to and from land, rivers, and the ocean. Sediment-associated biota are integral to these functions. Functional groups considered essential to CTZ processes include heterotrophic bacteria and fungi, as well as many be nthic invertebrates. Key invertebrate functions include shredding, which br eaks down and recycles organic matter; suspension feeding, which collects a nd transports sediments across the sediment-water interface; and bioturbati ng, which moves sediment into or out of the seabed. In addition, macrophyte s regulate many aspects of nutrient, particle, and organism dynamics above- and belowground. Animals moving within or through CTZs are vectors that tr ansport nutrients and organic matter across terrestrial, freshwater, and ma rine interfaces. Significant threats to biodiversity within CTZs are posed by anthropogenic influences; eutrophication, nonnutrient pollutants, specie s invasions, overfishing, habitat alteration, and climate change affect spe cies richness or composition in many coastal environments. Because biotic d iversity in marine CTZ sediments is inherently low whereas their functional significance is great, shifts in diversity are likely to be particularly i mportant. Species introductions (from invasion) or loss (from overfishing o r habitat alteration) provide evidence that single-species changes can have overt, sweeping effects on CTZ structure and function. Certain species may be critically important to the maintenance of ecosystem functions in CTZs even though at present there is limited empirical evidence that the number of species in CTZ sediments is critical. We hypothesized that diversity is indeed important to ecosystem function in marine CTZs because high diversit y maintains positive interactions among species (facilitation and mutualism ), promoting stability and resistance to invasion or other forms of disturb ance. The complexity of interactions among species and feedbacks with ecosy stem functions suggests that comparative (mensurative) and manipulative app roaches will be required to elucidate the role of diversity in sustaining C TZ functions.