Genetic structure of eelgrass (Zostera marina) within Humboldt Bay, California

Thesis (M.S.)--Humboldt State University, Biology, 2014Genetic structure and diversity of populations can impact the productivity and resilience of ecosystems. In Humboldt Bay, California (USA) eelgrass (Zostera marina) is the dominant marine plant at low intertidal and shallow subtidal elevations where it provides many ecological functions. However, its genetic makeup in Humboldt Bay has never been described. This study tested the hypotheses that eelgrass genetic diversity and structure differ across vertically and horizontally arranged subpopulations within Humboldt Bay. Ten microsatellite markers were examined for each of the 265 samples (i.e., shoots) collected across horizontal strata. The vertical subpopulations were located only in the North and South Bay portions of Humboldt Bay and included 165 samples from the mid intertidal (HIGH), low intertidal (LOW) and the shallow subtidal (SUB). The horizontal subpopulations included 32 samples from Mad River Slough (MRS), 93 from North Bay (HBN), 34 from Freshwater Slough (FWS), 34 from Elk River (ELK), and 72 from South Bay (HBS). AMOVA, Bayesian analyses, and Discriminant Analyses of Principle Components (DAPC) failed to reveal significant genetic structure among the horizontal or vertical populations. Global FST and overall RST values were close to zero implying that the subpopulations in both datasets share many alleles in similar frequency. Pairwise FST comparisons of MRS:ELK and MRS:HBS were slightly significant indicating that there are alleles that are not shared in equal frequency between these subpopulations. Although not statistically significant, some distinctive structure of ELK and MRS was observed in scatterplots in DAPC: this is consistent with their physical isolation within the bay and ecological attributes of each system. In contrast to the lack of structural variability among subpopulations, a large proportion of variability was identified among samples as demonstrated by high levels of clonal richness (R > 0.97), overall heterozygosity (HO = 0.51, HE = 0.53) and allelic richness (5.6 - 7.4). These findings could be due to a combination of plant (e.g., outcrossing) and environmental attributes (e.g., too few generations) that minimize genetic differentiation in the bay. Alternatively, HBN may have lost genetic structure due to disturbances accumulated since 1850 and HBN may be relying on seed dispersal from other areas of the bay, thereby minimizing horizontal genetic structure