Production and vertical distribution of invertebrates on riprap shorelines in Chesapeake Bay: A novel rocky intertidal habitat

Abstract

Abstract Along the North American Mid-Atlantic coast, soft-sediment shorelines are armored with hard substrata, such as riprap (rock revetments), for erosion protection. These structures are valuable for determining anthropogenic effects on marine ecosystems, and riprap potentially represents a habitat similar to an “emerging ecosystem” with productive species assemblages. The goals of this project were to (i) quantify species diversity, secondary production, and vertical distribution associated with armored (i.e., riprap) habitats in various locations in Chesapeake Bay and (ii) compare benthic epifaunal production on riprap with infaunal production on natural soft-sediment habitats in a system where natural rocky intertidal habitats are absent. Density, diversity, and secondary production of benthic epifauna on riprap and benthic infauna were compared in adjacent habitats in lower (Lynnhaven River) and upper (Patuxent River) Chesapeake Bay. To compare riprap habitat between tributaries that differed in physical characteristics and with natural rocky intertidal habitats from other systems, oyster, mussel, and barnacle vertical distribution on riprap in southern (Lynnhaven River) and northern (Piankatank River) locations was quantified. The hypothesis was that riprap assemblages and secondary production would differ between rivers and among regions due to differences in physical variables such as temperature (thermal stress in northern locations), or osmotic stress (greater upriver), and patterns would be comparable to natural rocky habitats. We predicted that productivity would be relatively high and relate positively with diversity on riprap, suggesting that it is a habitat within an “emerging ecosystem” where new combinations of species and habitat appear. Benthic epifaunal production on riprap was sevenfold greater than natural infaunal production, and both tended to be highest in the Lynnhaven. Vertical distribution on riprap was similar to that on natural rocky intertidal habitats: oyster and mussel densities tended to be highest in the mid- and low-intertidal zones, whereas barnacle density was greatest in the high intertidal at the southern location and the low intertidal in the northern location. Secondary productivity on riprap was high relative to the natural, infaunal, soft-bottom habitat in this area. Species richness and epifaunal non-oyster production were positively related to oyster production. We demonstrate that intertidal riprap-armored shorelines are a productive novel habitat in an emerging ecosystem, and they enhance species diversity, abundance, and production. Thus, riprap shorelines mimic the ecological characteristics of natural hard-bottom habitats, and they may contribute substantially to ecosystem function throughout the Atlantic and Gulf of Mexico coasts due to riprap s widespread use and distribution.