Absence of recovery in a degraded eelgrass (Zostera marina L.) bed in Nova Scotia, Canada : results from a transplant study

Abstract

By the early 2000s, the invasion of the European green crab (Carcinus maenas) had contributed to severe decline of eelgrass (Zostera marina) beds in eastern Canada. Some eelgrass beds have recovered, but Benoit Cove, Nova Scotia has failed to rebound. The primary objectives of this study were to determine whether the cove has reached a state at which it can no longer return to a healthy eelgrass bed, to evaluate the economic importance of restoring eelgrass in Atlantic Canada, and to explore Canadian policies and management practices for seagrass protection. From 3 July – 29 August 2018, I conducted an eelgrass transplant experiment in Benoit Cove and the donor site, Tracadie Harbour, using a modification of the TERFS method. Transplant survival, rhizome growth, and above-ground growth (canopy height, blade length, number of blades per shoot, and blade width) were measured. Above-ground growth declined in both sites, but all blade-size variables were significantly smaller in Benoit Cove. Negative transplant growth trends may have been tied to higher water temperature and epiphytic algal cover. Tracadie Harbour had a final transplant survival rate of 91.6% which was significantly greater than Benoit Cove (15.4%). Sediment composition, organic matter, and associated biota were also evaluated. The sediment from both sites was composed mainly of silt (> 28%). It is probable that the absence of eelgrass in Benoit Cove induced sediment resuspension and turbidity. The combination of turbidity and estimated drift algal cover (50–75%) may have increased light attenuation to a point at which the transplants could no longer tolerate their surroundings. The biota survey confirmed that Benoit Cove no longer has the faunal community of a typical eelgrass system. Tracadie Harbour was more species-rich and had larger populations of fish and meiofauna, which is expected in seagrass habitats. These species were essentially absent in Benoit Cove; instead, species of molluscs and annelids dominated. The number of green crabs counted in Benoit Cove was roughly 0.01 crabs mˉ², which was lower than the recorded number in 2013 (0.03 crabs mˉ²). Tracadie Harbour had a significantly higher green crab count (0.075 crabs mˉ²), suggesting that green crab populations are influenced by the presence of dense eelgrass beds. Atlantic Canada has an economic dependence on commercial fisheries and many of these species depend on the support of Z. marina beds at some point in their life. Therefore, the benefits of monitoring and investing in eelgrass restoration can certainly outweigh the costs. For several decades, eelgrass populations have suffered from lax policies and management practices in Eastern Canada. Considering the increasing anthropogenic activities that are negatively affecting coastal habitats, I hereby recommend that Canada establishes strict policies aimed directly at protecting valuable eelgrass habitat, improves data collection and monitoring of Z. marina, and increases social awareness and community involvement.