Interactions between seagrass complexity, hydrodynamic flow and biomixing alter food availability for associated filter-feeding organisms.

Vanessa González-Ortiz,José L Pérez-Lloréns,Luis G Egea,Fernando G Brun,Tjeed J Bouma,Francisco Moreno-Marín,Rocio Jiménez-Ramos,John F Valentine

doi:10.1371/journal.pone.0104949

Vanessa González-Ortiz, José L Pérez-Lloréns + Show 6 more

Open Access

https://doi.org/10.1371/journal.pone.0104949

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Abstract

Seagrass shoots interact with hydrodynamic forces and thereby a positively or negatively influence the survival of associated species. The modification of these forces indirectly alters the physical transport and flux of edible particles within seagrass meadows, which will influence the growth and survivorship of associated filter-feeding organisms. The present work contributes to gaining insight into the mechanisms controlling the availability of resources for filter feeders inhabiting seagrass canopies, both from physical (influenced by seagrass density and patchiness) and biological (regulated by filter feeder density) perspectives. A factorial experiment was conducted in a large racetrack flume, which combined changes in hydrodynamic conditions, chlorophyll a concentration in the water and food intake rate (FIR) in a model active filter-feeding organism (the cockle). Results showed that seagrass density and patchiness modified both hydrodynamic forces and availability of resources for filter feeders. Chlorophyll a water content decreased to 50% of the initial value when densities of both seagrass shoots and cockles were high. Also, filter feeder density controlled resource availability within seagrass patches, depending on its spatial position within the racetrack flume. Under high density of filter-feeding organisms, chlorophyll a levels were lower between patches. This suggests that the pumping activity of cockles (i.e. biomixing) is an emergent key factor affecting both resource availability and FIR for filter feeders in dense canopies. Applying our results to natural conditions, we suggest the existence of a direct correlation between habitat complexity (i.e. shoot density and degree of patchiness) and filter feeders density. Fragmented and low-density patches seem to offer both greater protection from hydrodynamic forces and higher resource availability. In denser patches, however, resources are allocated mostly within the canopy, which would benefit filter feeders if they occurred at low densities, but would be limiting when filter feeder were at high densities.

Highlights

Seagrasses are important ecosystem engineers, which can change the physical environment through their physical structures [1]
The specific objectives of our study were: (1) to establish the relationship between hydrodynamics and seagrass complexity, (2) to measure how variations in flow characteristics alter the availability of resources for filter feeders, (3) to estimate if flow and food concentration were modified by the presence of high densities of an active filter feeder, and (4) to check whether the interaction of both seagrass
A well-formed TKE wake behind the first patch was observed, especially in the high shoot density treatments (Hbbb and HbbH)

Summary

Introduction

Seagrasses are important ecosystem engineers, which can change the physical environment through their physical structures [1] Such habitat modification can result in positive feedbacks, stabilizing seagrass meadows [2] as well as having either a positive (e.g. facilitation) or negative effects on the survival of associated species [3,4]. Other studies have pointed out in turn that the reduction of particle fluxes associated with attenuated conditions within the canopy could fully deplete resources within the bottom benthic boundary layer and negatively affect the growth rates of filter feeders [8,11,14,30,31]

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