A halo of reduced dinoflagellate abundances in and around eelgrass beds.

Emily Jacobs-Palmer,Micah Horwith,Ryan P Kelly,Emily Kunselman,Kelly Cribari,Ana Ramón-Laca,Ramón Gallego

doi:10.7717/peerj.8869

Abstract

Seagrass beds provide a variety of ecosystem services, both within and outside the bounds of the habitat itself. Here we use environmental DNA (eDNA) amplicons to analyze a broad cross-section of taxa from ecological communities in and immediately surrounding eelgrass (Zostera marina). Sampling seawater along transects extending alongshore outward from eelgrass beds, we demonstrate that eDNA provides meter-scale resolution of communities in the field. We evaluate eDNA abundance indices for 13 major phylogenetic groups of marine and estuarine taxa along these transects, finding highly local changes linked with proximity to Z. marina for a diverse group of dinoflagellates, and for no other group of taxa. Eelgrass habitat is consistently associated with dramatic reductions in dinoflagellate abundance both within the contiguous beds and for at least 15 m outside, relative to nearby sites without eelgrass. These results are consistent with the hypothesis that eelgrass-associated communities have allelopathic effects on dinoflagellates, and that these effects can extend in a halo beyond the bounds of the contiguous beds. Because many dinoflagellates are capable of forming harmful algal blooms (HABs) toxic to humans and other animal species, the apparent salutary effect of eelgrass habitat on neighboring waters has important implications for public health as well as shellfish aquaculture and harvesting.

Highlights

Seagrass species are ecosystem engineers throughout the world’s coastal zones (Jones, Lawton & Shachak, 1994), generating and sustaining habitat for a multitude of associated taxa (Duffy, 2006)
An example plot of samples gathered along the transect from eelgrass to bare substrate at Willapa Bay in July (Fig. 2; all site/date plots shown in Fig. S7) shows that the eelgrass community is quite dissimilar from other transect points along both axes
For the instances in which complete transects were sampled at a given time and place (10) and all three technical replicates of a sample were available for analysis (60), 44 samples (73%) were non-overlapping in ordination with all remaining transect points, demonstrating that despite proximity at the scale of meters, bottles of water contained environmental DNA (eDNA) evidence of distinct biological communities the majority of the time

Summary

Introduction

Seagrass species are ecosystem engineers throughout the world’s coastal zones (Jones, Lawton & Shachak, 1994), generating and sustaining habitat for a multitude of associated taxa (Duffy, 2006). These marine macrophytes provide a wide variety of essential ecosystem services that directly benefit humans, such as provision of nursery habitat for food species (Heck, Hays & Orth, 2003) and coastal protection through sediment accretion and stabilization (Potouroglou et al, 2017; reviewed in Nordlund et al (2016)). Seagrass habitat is a globally important resource, with far-reaching positive economic effects

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