| Summary: | Blue mussels (Mytilus spp.) are important ecosystem bioengineers and provide numerous of ecosystem services. They are found worldwide, but face a global population decline for unclear reasons in many regions. A pilot study in the subarctic rocky intertidal shore in Northern Norway revealed an unexpected unimodal pattern in the body size of blue mussels along a vertical transect from low to high shore. Here, the size of blue mussels increased with shore height, contradicting ecological theories and other studies where mussel size decreased with shore height. It is well-established that size is a known trait proximately enhancing survival and reproduction, thereby influencing population dynamics. This study aims to validate and elucidate the processes behind the unusual size pattern through five field experiments conducted from February to August 2023. (1) First, two open-coast and two inner-fjord sites were selected, replicating the pilot study transects, where mussel size and density were recorded. (2) Recruitment distribution across the different intertidal zones was examined. (3) A mussel-transplantation experiment between high and low intertidal zones was conducted to quantify blue mussel growth. (4) To explore density-dependent effects on mussel growth, a mussel-patch manipulation experiment was performed to test whether reduced density enhanced their growth. Finally, (5) an assessment of the coverage of potential predators, competitors, and facilitators was conducted along the transects. Results showed that the unexpected pattern is limited to open-coast sites, and absent at the inner-fjord sites, suggesting local variation in mussel size patterns across intertidal transects. Growth conditions were most favorable in the low intertidal zone (effect size = 0.81), while recruit density was not statistically different across the intertidal zones. Dogwhelks (Nucella lapillus) emerged as the primary contributor to mussel mortality (≈70%) in the low intertidal, in addition to exhibiting a feeding preference for ...
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