Creating wetland islands to enhance shoreline habitat for fish recruitment in a modified shallow lake

Abstract

Soft shoreline engineering is increasingly used to combine shoreline fortification with the enhancement of biodiversity and biological production of land–water transitions. From 2016 to 2021, the large‐scale ecosystem restoration project Marker Wadden has created new multiple wetland islands from local sediments in the highly modified Lake Markermeer, the Netherlands. Instead of replacing steep rip‐rap shorelines with soft shorelines, new islands with soft land–water transitions were engineered to offset the marked declines in bird and fish populations in this Natura 2000 area, protected under the European Union Birds and Habitats Directives. This new approach was evaluated by assessing the added value of the newly created wetland islands with soft shorelines to the existing steep rip‐rap fortified shores of the lake, for the enhancement of fish spawning and nursery habitat. Young‐of‐the‐year fish densities at the Marker Wadden islands were highest in sheltered bays and wetlands with nutrient‐rich silt sediments, and lowest at wind‐exposed sandy beaches. Both newly engineered soft shorelines and existing rip‐rap shorelines contributed to habitat diversity, although fish densities declined considerably with increasing exposure to wind‐induced wave power. Building soft shorelines as a new archipelago instead of replacing existing shoreline habitats increased the total length of the land–water transitions in the lake. The 800 ha Marker Wadden archipelago covers only 1% of the 70,000 ha lake surface area, but provides a 16% increase in shoreline habitat and a fivefold increase in soft shoreline for the lake. We conclude that designing wetland islands as a means of lake restoration can contribute effectively to sheltered habitat enhancement for fish spawning and nurseries, and thereby to the potential conservation of fish communities. The approach of building islands achieves this without compromising the complementary functionality of the original, more mature, shorelines of the lake.