Date of Award

2002

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Oceanography

First Advisor

Charles Roman

Abstract

Salt marsh pools are shallow, steep-sided depressions that remain flooded throughout a tidal cycle and provide important habitat for nekton (fish and decapod crustaceans). Although New England pools are relatively unstudied, efforts are underway to expand pool habitat through an open marsh water management technique know as ditch plugging. The purpose of this study was to quantify pool geomorphic and geographic characteristics, to determine distribution of vegetative cover types and nekton species, and to evaluate effects of ditch plugging on nekton use of pools.

Over 30 ditched and unditched marshes were surveyed from the Connecticut shore of Long Island Sound to southern Maine, USA for pool physical traits. Pools from ditched and unditched marshes had similar sizes, depths and distances on tidal flow, but not pool density (#/ha marsh) and coverage (m² pool/ha marsh) was negatively correlated to pool density and coverage.

Pool vegetative cover was surveyed at 12 New England marshes using 0.25m² quadrants and the Braun-Blanquet scale for coverage estimation. Eight cover types were identified with the most common being "bare" (pool bottom visible), filamentous green algae, algal flocculation and Ruppia maritima. Pool cover types varied by region, with environmental conditions supporting R. maritima and algal floc coverage were distinguished by water depth, soft sediment depth and water column nitrogen concentrations.

Pool nekton sampling using 1m² throws traps in 1999 and 2000 documented 12 fish and 4 decapod species with Fundulus heteroclitius comprising 80% of the fish catch. Species richness and density were greater in southern than in northern New England sites. In contrast to results from other studies, fish densities were not affected by the presence of submerged aquatic vegetation (R. maritima). Data suggested that F. Heteroclitius use pools as nursery habitat and as refuges from predation, selecting among pools with different environmental conditions as the season progressed and juvenile size increased. Restoration efforts, therefore, should maintain a range of pool conditions to support a diverse nekton community and its changing needs.

The effects of ditch plugging at 3 salt marshes in the Rachel Carson National Wildlife Refuge, Maine, USA, were evaluated using a BACI (Before-After-Control-Impact) study design. Throw trap sampling produced a total of 7 fish and 3 decapod species in salt marsh pools with F. heteroclitius accounting for 89% of the fish catch. After controlling for natural variation using the BACI design, nekton community and total fish density remained unaltered at Moody and Granite Point. Decreased species richness at Granite Point marsh was thought due to physical barriers created by ditch plugs and a 51% reduction in tdal interface. At Marshall Point, ditch plugging resulted in increased pool habitat and nekton use at the experimental versus control area pools. Both fish and decapods occupied newly created pools within weeks and within months new pools were not different from old pools in terms of nekton use. This study documents only initial nekton response to ditch plugging, longer term monitoring is necessary to ensure achieving management goals.

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