Eelgrass (<i>Zostera marina</i>) Food Web Structure in Different Environmental Settings

<div><p>This study compares the structure of eelgrass (<i>Zostera marina</i> L.) meadows and associated food webs in two eelgrass habitats in Denmark, differing in exposure, connection to the open sea, nutrient enrichment and water transparency. Meadow structure strongly reflected the environmental conditions in each habitat. The eutrophicated, protected site had higher biomass of filamentous algae, lower eelgrass biomass and shoot density, longer and narrower leaves, and higher above to below ground biomass ratio compared to the less nutrient-enriched and more exposed site. The faunal community composition and food web structure also differed markedly between sites with the eutrophicated, enclosed site having higher biomass of consumers and less complex food web. These relationships resulted in a column shaped biomass distribution of the consumers at the eutrophicated site whereas the less nutrient-rich site showed a pyramidal biomass distribution of consumers coupled with a more diverse consumer community. The differences in meadow and food web structure of the two seagrass habitats, suggest how physical setting may shape ecosystem response and resilience to anthropogenic pressure. We encourage larger, replicated studies to further disentangle the effects of different environmental variables on seagrass food web structure.</p></div

The relative (%) species contribution to total fish biomass (AFDW m^-2) in Dalby Bay (DB) and Kertinge Nor (KN) in June 2011.

<p>The relative (%) species contribution to total fish biomass (AFDW m^-2) in Dalby Bay (DB) and Kertinge Nor (KN) in June 2011.</p

Biomass distribution at trophic levels of <i>Zostera marina</i> meadows in Dalby Bay and Kertinge Nor, Denmark, June 2011.

<p>The biomass of each species is distributed across trophic levels (TL) 1–4 according to their diet, based on stable isotope mixing model results (Tables <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146479#pone.0146479.t005" target="_blank">5</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146479#pone.0146479.t006" target="_blank">6</a>), gut contents (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146479#pone.0146479.s003" target="_blank">S3 Table</a>), and literature data (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146479#pone.0146479.s004" target="_blank">S4 Table</a>). Combined width of bars indicate biomass at each TL in (A) Dalby Bay and (B) Kertinge Nor. Note biomass value of TL1 in KN is divided by 5 for visual purposes. (C) Linear regression of TL 2–4 and log₁₀biomass (DB: log₁₀biomass = 1.416–0.374×TL, R² = 0.99; KN: log₁₀biomass = 0.683 + 0.017×TL, R² = 0.24), with slope indicating overall shape of the consumer part of biomass pyramids in Fig 6A and 6B above.</p

Map and photos of the study areas on Funen Island, Denmark.

<p>Hatched areas indicate the studied eelgrass meadows in Dalby Bay (55°31ʹ07ʺ N, 10°37ʹ05ʺ E) and Kertinge Nor (55°26ʹ52ʺ N, 10°33ʹ30ʺ E) while photos illustrate differences in filamentous macroalgae, eelgrass shoot density, and jellyfish abundance. Encircled M shows location of monitoring stations for water column data. Contains data from the Map 10 data set of the Danish Geodata Agency, 2015.</p

Stable isotope values of species associated to <i>Zostera marina</i> meadows in Dalby Bay (DB) and Kertinge Nor (KN) in Denmark, June 2011.

<p>Mean δ¹³C vs. δ¹⁵N in (A) DB and (B) KN. Ascending mean δ¹⁵N values of consumer species in (C) DB and (D) KN. The dashed line indicates the convex hull. TA is the total area of the convex hull. <i>A</i>. <i>aurita</i> was excluded from the TA due its special position in the food web and lack of sampling in DB; See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146479#pone.0146479.t003" target="_blank">Table 3</a> for species abbreviations and isotopic signals.</p

<i>Zostera marina</i> meadow characteristics in Dalby Bay and Kertinge Nor.

<p>Values are mean ± SE. Asterisks indicate statistical significant differences based on t-tests (* = 0.05, *** = 0.001). For stacked columns the statistics refer to the summed value. (A) <i>Z</i>. <i>marina</i> and algal biomass. Numbers in columns are above to below ground biomass ratios. (B) <i>Z</i>. <i>marina</i> shoot density and leaf area index. (C) <i>Z</i>. <i>marina</i> above ground production.</p