Document Type

Report

Publication Date

11-2020

Pages

69 pp.

Abstract

Executive Summary

The subtidal macrobenthic communities off Swimming Point in the Elizabeth River were quantitatively sampled in summer 2019. The primary objective of this study was to characterize the ecological condition of the benthos compared to benthic data collected in summer 2010 (Dauer 2011).

The estimated the level of degraded benthic bottom of the Swimming Point stratum (BIBI < 3.0) in 2019 was 64% ± 18.8% a decrease from the 2010 value of 84% ± 14.0%. The level of severely degraded bottom (BIBI ≤ 2.0) in 2019 was 28% ± 17.6% compared to the 2010 value of 52% ± 19.6%. The ecological condition of benthic communities at Swimming Point in 2019 were comparable to the values for the Mainstem segment of the Elizabeth River in 2019 which had an estimated degraded benthic bottom of 52% ± 19.6% and a value of 36% ± 18.8 of severely degraded bottom.

The BIBI and its metrics showed a mixture of improvements and degradations in benthic community condition. Compared to the previous benthic community condition reported in Dauer (2011) the 2019 Swimming Point benthos: (1) the average BIBI value improved significantly from 2.2 ± 0.1 in 2010 to 2.6 ± 0.1 in 2019 (p = 0.04), (2) total community abundance also improved with a decrease from an over-abundance value of 6,508 ± 993 individuals per m2 in 2010 to 4,482 ± 482 individuals per m2 in 2010 (p = 0.08) – an improvement in ecological condition for polyhaline sand habitats, (3) a significant decrease in the Shannon Diversity Index value from 1.92 ± 0.15 in 2010 to 1.49 ± 0.11 in 2019 (p = 0.03), and (4) a significant decrease in the number of species per sample from 13.6 ± 0.7 in 2010 to 9.4 ± 0.6 in 2019 (p < 0.01).

A comparison of the temporal changes in benthic community condition at Swimming Point (2010 data to 2019 data) to the patterns throughout the Elizabeth River (1999 data compared to 2019) in Dauer (2020) shows (1) the BIBI values significantly improved only in the Southern Branch and at Swimming Point with the highest BIBI values in the Mainstem of the river and at Swimming Point, (2) the BIBI values decreased in the Lafayette River, Eastern Branch and Western Branch in 2019, (3) abundance values increased in the Lafayette River, the Eastern Branch and the Western Branch, above or near the over-abundance threshold of 5,000 individuals per m2 while significantly declining below the threshold at Swimming Point, and (4) the decrease in the Shannon Diversity Index and species richness at Swimming Point also occurred in the Lafayette River, the Eastern Branch and the Western Branch.

Indicative of improved benthic ecological condition was the appearance in the density dominants of 2019 of (1) two gastropod species - the glassy bubble shell Haminoea solitaria and the barrel bubble shell Acteocina canaliculate, (2) the bivalve Macoma balthica, (3) the amphipod Ampelisca abdita, and the shrimp Ogyrides alphaeorostris. All these species are generally associated with unstressed estuarine and/or coastal habitats.

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Included with kind permission from the author(s).

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