Distribution of Zostera marina L. and Ruppia maritima L. sensu lato along depth gradients in the lower Chesapeake Bay, U.S.A.

Abstract

Seventeen transects in areas containing beds of submerged aquatic vegetation in the lower Chesapeake Bay were selected for analysis of the depth distribution of Ruppia maritima L. sensu lato and Zostera marina L. during a 6-week period (25 July–12 September 1978). Transects studied ranged in length from 130 to 1100 m with estimates of percent cover made on 785 plots. Mean importance value (relative frequency + relative cover) for Z. marina was 96.0 (range of 0–200) while for R. maritima it was 94.9 (0–184.4). Average cover across the beds ranged from 0 to 51.7% for Z. marina and 0–79.2% for R. maritima, with average transect biomass up to 72.9 and 55.4 g dry weight m −2 for the two species, respectively. Comparison of individual transects showed a consistent pattern of zonation where R. maritima occupied the nearshore, shallower area which graded to a mixed zone of R. maritima and Z. marina at intermediate depths. At the deepest part of the beds, Z. marina was the only species found. Transects along the western shore sites were characterized by lower percent cover with more open areas in the beds when compared with the eastern shore sites. Depth distributions of R. maritima and Z. marina on the eastern shore were +20 to −100 cm and −30 to −150 cm (mean low water (MLW)), respectively, while on the western shore they were +10 to −80 cm and +10 to −110 cm, respectively. The greater depth penetration of the two species along the eastern shore transect sites may reflect a greater influence of clearer, oceanic water compared with the more turbid, riverine influence along the western shore sites. Results demonstrate that both optimum and maximum depth limits for a species can vary considerably within a particular region and suggest the potential for marked variability over time.