Severe shifts of Zostera marina epifauna: comparative study between 1997 and 2018 on the Swedish Skagerrak coast

Close-up photo of a small gastropod sitting on a Zostera marina eelgrass leaf in Gullmarsfjord, Swedish west coast.

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Journal Papers

Riera R, Vasconcelos J, Baden S, Gerhardt L, Sousa R, Infantes E

Marine Pollution Bulletin, 158: 111434

Publication year: 2020

Severe shifts of Zostera marina epifauna: Comparative study between 1997 and 2018 on the Swedish Skagerrak coast

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Abstract

The interaction between bottom-up and top-down processes in coastal ecosystems has been scarcely studied so far. Temporal changes in trophic interactions of Zostera marina along the Swedish west coast are relatively well studied, with the exception of epifaunal communities. Epifauna was used as a model study to explore resource (bottom-up) or predator (top-down) regulated in a vegetated ecosystem.

We conducted a 21-year comparative study (1997 and 2018) using epifauna of 19 Zostera marina meadows along the Swedish Skagerrak coast. Large changes were observed in the composition of small (0.2–1 mm) and large (>1 mm) epifauna. In the small-sized epifauna, the nematode Southernia zosterae and harpacticoids showed an increase of 90% and a decrease of 50% of their abundances, respectively. In the large-sized epifauna, the polychaete Platynereis dumerilii and chironomid larvae were absent in 1997 but thrived in 2018 (>2000 ind. m⁻²). Mesoherbivores (Idoteids and gammarids) were locally very abundant in 1997 but disappeared in 2018.

An 83% decline of mytilids settling in Zostera marina leaves was observed. Our results showed that epifauna is predominantly top-down regulated. An integrative framework of the study area is outlined to shed light on the causes and consequences of the environmental shifts reported in Zostera meadows from the northern Skagerrak area throughout the last three decades.

https://doi.org/10.1016/j.marpolbul.2020.111434

Highlights

The interaction between bottom-up and top-down processes in coastal ecosystems was studied along the Swedish west coast.
A 21-year comparative study of epifauna in 19 Zostera marina (eelgrass) meadows was conducted.
Large changes were observed in the composition of small and large epifauna, with some species thriving and others declining.
Results suggest that epifauna in the study area is mainly regulated by top-down processes. An integrative framework is outlined to investigate the environmental shifts reported in eelgrass meadows.

Riera Severe shifts of Zostera marina epifauna: Comparative study between 1997 and 2018 on the Swedish Skagerrak coast

Fig. 7. Conceptual model of the regime shift observed in the Z. marina meadows between 1997 and 2018 in the Skagerrak. The percentage changes found within each group (circle) is shown with the largest circle being 100% and the smaller circle is a percentage area of the 100% circle. The low densities of top predators, e.g. cod (Gadus morhua) (Baden et al., 2012; Sandström et al., 2018) directly exert control on the abundances of intermediate predators dominated by sticklebacks (Gasterosteus aculeatus), gobies (Gobius niger and Gobiusculus flavescens) and Palaemon adspersus (Baden et al., 2012). The fish species preferably preyed on mesograzers (Gammarus spp. and idoteid isopods) in 1997 but turn to small amphipods, harpacticoids and nereidids in 2018. Mesograzers are pivotal to control overgrowth of filamentous algae on Z. marina leaves (Svensson et al., 2012). The densities of mesograzers greatly affect the structure of large epifauna, with a good representation of sessile filter feeders in 1997 (e.g. the anemone Sagartiogeton viduatus and the ascidian Ciona intestinalis) and generalist vagile species in 2018 (the polychaete Platynereis dumerilii and chironomid larvae) that may prey on small epifauna, mainly nematodes + harpactioids. The current contribution of both epifaunal fractions (large and small) to maintain low levels of filamentous algae seems to be minor compared to grazing pressure of mesograzers in 1997.