Flow-Driven Variation in Intertidal Community Structure in a Maine Estuary

Abstract

Understanding the factors that determine community structure remains one of the most important issues in ecology. In this paper, we examine the role of flow velocities in governing community structure in marine intertidal communities. We broaden the traditional definition of ''bottom-up'' forces to include the delivery of nutrients, food, and larval resources to habitats and then test the hypothesis that, by controlling these fluxes as well as mediating predator effects, flow velocities leave strong bottom-up signatures on shoreline communities.We examined this hypothesis by quantifying community structure and dynamics at high and low flow sites in a tidal estuary in Maine. High flow sites were characterized by dense barnacle and mussel cover, while low flow sites had considerable bare space. High flow sites also had greater grazer and predator densities than low flow sites. Recruitment of all common shoreline organisms with planktonic larvae was greater at high flow sites, in direct proportion to the increased fluid flux there. Flow effects on the growth of the herbivorous and carnivorous components of the food web were less predictable. High flows increased the growth of barnacles, but not mussels, and increased the growth of the carnivorous gastropods that preyed on them. In contrast, high flows decreased the accumulation of benthic diatoms, but this was unrelated to the growth rates of herbivorous gastropods. High flow sites were universally characterized by low predation intensity and per capita predation rates on all three prey species.Our results show that the strength of top-down and bottom-up forces varies with flow rate in this estuary. Consumer stress (top-down) models accurately explain patterns we saw at low flow sites, but bottom-up processes predicted from nutrient/productivity models dominate at high flow sites. High consumer pressure is the dominant structuring force at low flow sites, whereas at high flow sites predators are inhibited, and individual recruitment and growth rates become the dominant structuring forces. We suggest that hydrodynamics may commonly decouple predation and resource processes in many aquatic systems when the physical process responsible for variation in top-down forces also acts as a strong bottom-up force.

A trophic cascade regulates salt marsh primary production

“…At both study sites, juvenile snail densities were nearly 300% higher in the tall-form zone (P Ͻ 0.01, two-way ANOVA, for main effect of zone, P Ͻ 0.001, Tukey test, x density in tall-form zone ϭ 752 Ϯ 34.5 ind per m 2 ; in short-form zone ϭ 205 Ϯ 12.4 ind per m 2 ). This finding suggests that snail recruitment is much greater in the low marsh, probably because of high larval fluxes on the edges of tidal creeks (20). Moreover, snail transplant experiments showed that Littoraria also grows best (Ϸ200% greater) in the lowest reaches of the marsh (P Ͻ 0.02, two-way ANOVA, for main effect of zone, P Ͻ 0.001, Tukey test, x change in shell length for tall-form zone ϭ 6.3 Ϯ .31 mm; for short-form zone ϭ 2.2 Ϯ .12 mm), which is most likely the result of decreased desiccation stress and͞or increased food quality (i.e., plants with higher N content; refs.…”

Gypsum dissolution is not a universal integrator of ‘water motion’

“…Gaylord (1999) measured wave periods of 8 to 18 s in a rocky intertidal location. Gypsum-dissolution techniques are often used in the intertidal zone (e.g., Sanford et al 1994;Leonard et al 1998), and wave period should be considered here as an additional measure that may affect mass-transfer rates. Wave period is directly related to wave frequency (Wave period ϭ 1/wave frequency).…”

“…Many studies of physical-biological interactions that use dissolution techniques do not address mass-transfer problems, but address questions that depend directly on flow estimates. Such questions include particle transport to animals on reefs or near rocks and animal growth (Cusson and Bourget 1997;Bartol et al 1999), the effect of flow on plant or animal morphology (Gerard and Mann 1979;Molloy and Bolton 1996), or the effect of flow on community composition (Cusson and Bourget 1997;Leichter and Witman 1997;Leonard et al 1998) (Table 1, column D). Often no relation is found between flow measured using dissolution techniques and the biological variable of interest, although there might be a relationship if a more reliable measure of flow was used (e.g., Newsome and Aalto 1987;Bartol et al 1999).…”

Recruitment, abundance, and predation on the blue mussel (Mytilus edulis) on northeastern estuarine rocky shores

“…Station name Leonard et al 1998). Each collector (n ¼ 8 per site) was secured with a lag bolt and washer in the mid intertidal zone.…”

“…As filter feeding primary consumers, mussels link benthic and pelagic ecosystems and create habitat for many other species (Suchanek 1981, Witman et al 2003. Investigations into how key life history traits in mussels (such as physiology, growth, survival, and/or reproduction) vary with exogenous factors including temperature (Helmuth and Hof-mann 2001, Petes et al 2007, Lesser et al 2010, hypoxia (Altieri and Witman 2006), food availability (Menge et al 1997, Leonard et al 1998, and disturbance regimes Levin 1981, Hunt andScheibling 2001) have contributed substantially to understanding how environmental factors influence organismal and population performance, and community dynamics more broadly. Observations and experiments of predator-prey dynamics involving mussels have also yielded important insights, including understanding of how spatial and temporal variation in predation strength and predator identity can shape benthic community structure and functioning (Paine 1966, Robles et al 1995, Navarrete and Menge 1996, Menge et al 2004, Navarrete and Manzur 2008, Boudreau and Hamilton 2012.…”