Detecting the impacts of notorious invaders: experiments versus observations in the invasion of eelgrass meadows by the green seaweed Codium fragile

Abstract

Biological invasions can vary in the extent of their effects on indigenous communities but predicting impacts for particular systems remains difficult. In coastal marine ecosystems, the green seaweed Codium fragile ssp. fragile is a notorious invader with its reputation based on studies conducted largely on rocky shores. The green seaweed has recently invaded soft-bottom eelgrass communities by attaching epiphytically to eelgrass (Zostera marina) rhizomes, thereby creating the potential for disruption of these coastal habitats through competition or disturbance. We investigated the effect of this invader on various aspects of eelgrass performance (shoot density and length, shoot growth, above- and below-ground biomass, carbohydrate storage) using both small-scale manipulative and large-scale observational experiments. Manipulative experiments that varied Codium abundance demonstrated clear negative effects over a 4-month period on shoot density and carbohydrate reserves, but only for high, but realistic, Codium biomass levels. Light levels were much lower under canopies for high and medium density Codium treatments relative to low and control Codium cover treatments, suggesting that shading may influence eelgrass growing under the algal cover. In contrast, these effects were either not detectable or very weak when examined correlatively with field surveys conducted at larger spatial scales, even for sites that had been invaded for over 4 years. It is premature to extend generalizations of Codium's impact derived from studies in other systems to eelgrass communities; further efforts are required to assess the long-term threats that the alga poses to this ecosystem. This study demonstrates the need to investigate impacts of invasions over multiple scales, especially those that incorporate the temporal and spatial heterogeneity of the invader's abundance.

Phylogenetic Differences in Attached and Free-Living Bacterial Communities in a Temperate Coastal Lagoon during Summer, Revealed via High-Throughput 16S rRNA Gene Sequencing

“…Attached bacteria produce exoenzymes that target specific complex molecules, and the availability and stoichiometry of particulate organic matter likely selects for bacteria with specific degradation systems (94). Similar to other coastal lagoons (20), the eelgrass Zostera marina occurs in the shallow regions of the Magdalen Islands lagoons (95)(96)(97). Zostera marina increases in biomass over the summer growing season, consistent with a seasonal contribution to POM; maximum foliar biomass is in August, and stem density increases from July to September (98).…”

Contrasting seasonal and spatial distribution of native and invasive Codium seaweed revealed by targeting species‐specific eDNA

“…The ex situ experiment identified an upper limit of detection due to the selection of 1 ng/µl as the highest level of the standard curve dilution series in order to be able to detect low eDNA densities, representative for the values found in environment. Yet, the upper limit of detection was reached at concentrations that were unlikely to be found in the natural environment (Drouin, McKindsey, & Johnson, 2012;Scheibling & Gagnon, 2006). Primer specificity is important for successful detections of species (Mächler et al, 2014;Wilcox et al, 2013), but comparison of species densities using species-specific primers should be interpret with caution, comparing eDNA efficiency with the traditional abundance estimates (Agersnap et al, 2017).…”

Differential effects of biological invasions on coastal blue carbon: A global review and meta‐analysis

“…By contrast, the net effect of animal invasions was a reduction (−48% ± 27%) in blue C pools, largely caused by herbivory, trampling, and borrowing that removed vegetation or destabilized habitat (Bertness, 1984;Davidson & De Rivera, 2010;Sharp & Angelini, 2016). Similarly, a significant reduction in C storage (−37% ± 34%) resulted from structurally distinct primary producer invasions, mainly triggered by displacement or reduced density of native seagrasses by seaweeds and mangroves by marsh grasses (Ceccherelli & Campo, 2002;Drouin, McKindsey, & Johnson, 2012;Feng, Ning, Zhu, & Lin, 2017;Zhang, Huang, Wang, Chen, & Lin, 2012). These effect directions and sizes improve our understanding of C storage response to invasion in marine systems and provide further insight and some alignment with similar examinations of plant and animal invasion on C pools in terrestrial systems (Liao et al, 2008;Qiu, 2015).…”

“…In either scenario, loss of CVH species occurs without replacement by native or invading species that reengineer the same habitat, causing apparent reductions in the C pool. In seagrass systems, a range of algal invaded sites-by Caulerpa taxifolia, Caulerpa racemosa, Codium fragile, Gracilaria vermiculophylla, Lophocladia lallemandiiwere found to have reduced biomass or C storage compared to uninvaded meadows (Ballesteros, Cebrian, & Alcoverro, 2007;Ceccherelli & Campo, 2002;Drouin et al, 2012;Thomsen, Staehr, Nejrup, & Schiel, 2013;Williams & Grosholz, 2002). Likewise, the rich C pool that occurs in Chinese mangrove systems is being reduced by incursions of invasive Spartina alterniflora (Feng et al, 2017;Zhang et al, 2012).…”