Landscape setting affects the structure and function of oyster reefs
Habitat transition zones, or ecotones, are common among estuarine ecosystems. The connectivity between habitats is an important driver of population vital rates and community composition of mobile fauna. Formally, oyster reefs and salt marsh habitat were highly connected features of estuaries, but over decades of natural and anthropogenic stressors, the connectivity between marsh and fringing oyster reefs has been reduced and even lost. Understanding these interhabitat processes has broad implications for how ecosystems can be restored to enhance resilience and facilitate secondary (fisheries) production. Across three distinct marsh morphologies we asked: 1) does landscape setting affect the development of restored oyster reefs? and 2) does landscape setting affect the community composition of reef associated nekton? We constructed replicated oyster reefs along marsh shorelines defined as scarp, ramp, and creek. We sampled these restored reefs, non-restored reference sites (i.e. experimental controls) and naturally existing oyster reefs during five dates between Fall 2012 and Fall 2014. We sampled oyster populations and associated fauna within quadrats while mobile nekton were sampled via gill and fyke nets.
Oyster settlement and survival varied among landscapes. After initial settlement, we found significantly more oysters on the ramp landscape (compared to scarp and creek). However, in following years, we found a reversed trend where the greatest oyster densities occurred in the creek landscape. Two years after reef construction we found significantly higher densities on restored sites relative to controls, regardless of landscape context, though these densities were still lower than on natural reefs. Additionally, we consistently found a greater abundance of mobile fauna in the creek landscape, regardless of reef restoration. Therefore, we hypothesize that physical stress (wave energy) is moderating oyster production along ramp and scarp landscapes rather than strong top-down control via predation. Oyster reef presence (natural or restored) had no effect on mobile nekton potentially because the home range of the organisms was larger than the scale of the reefs. While landscape can affect the structure of biogenic reef habitat it does not seem to control the functional value of oyster reefs as nekton habitat. These data indicate that while landscape context may affect the fitness of foundation species such as oysters, the interactive effects between structural habitat complexity and landscape context on mobile nekton densities are not guaranteed.