Persistent differences between coastal and offshore kelp forest communities in a warming Gulf of Maine

Abstract

Kelp forests provide important ecosystem services, yet coastal kelp communities are increasingly altered by anthropogenic impacts. Kelp forests in remote, offshore locations may provide an informative contrast due to reduced impacts from local stressors. We tested the hypothesis that shallow kelp assemblages (12–15 m depth) and associated fish and benthic communities in the coastal southwest Gulf of Maine (GOM) differed significantly from sites on Cashes Ledge, 145 km offshore by sampling five coastal and three offshore sites at 43.0 +/- 0.07° N latitude. Offshore sites on Cashes Ledge supported the greatest density (47.8 plants m2) and standing crop biomass (5.5 kg m2 fresh weight) of the foundation species Saccharina latissima kelp at this depth in the Western North Atlantic. Offshore densities of S. latissima were over 150 times greater than at coastal sites, with similar but lower magnitude trends for congeneric S. digitata. Despite these differences, S. latissima underwent a significant 36.2% decrease between 1987 and 2015 on Cashes Ledge, concurrent with a rapid warming of the GOM and invasion by the kelp-encrusting bryozoan Membranipora membranacea. In contrast to kelp, the invasive red alga Dasysiphonia japonica was significantly more abundant at coastal sites, suggesting light or dispersal limitation offshore. Spatial differences in fish abundance mirrored those of kelp, as the average biomass of all fish on Cashes Ledge was 305 times greater than at the coastal sites. Remote video censuses of cod (Gadus morhua), cunner (Tautaogolabrus adspersus), and pollock (Pollachius virens) corroborated these findings. Understory benthic communities also differed between regions, with greater abundance of sessile invertebrates offshore. Populations of kelp-consuming sea urchins Stronglyocentrotus droebachiensis, were virtually absent from Cashes Ledge while small urchins were abundant onshore, suggesting recruitment limitation offshore. Despite widespread warming of the GOM since 1987, extraordinary spatial differences in the abundance of primary producers (kelp), consumers (cod) and benthic communities between coastal and offshore sites have persisted. The shallow kelp forest communities offshore on Cashes Ledge represent an oasis of unusually high kelp and fish abundance in the region, and as such, comprise a persistent abundance hotspot that is functionally significant for sustained biological productivity of offshore regions of the Gulf of Maine.

Temperature, phenology, and turf macroalgae drive seascape change: Connections to mid‐trophic level species

“…This is similar to other fish studies within the Gulf of Maine (Witman and Lamb 2018). This is similar to other fish studies within the Gulf of Maine (Witman and Lamb 2018).…”

Impacts of ocean warming on kelp forest ecosystems

“…Kelp forests in the NW Atlantic Ocean have also responded to recent warming trends, perhaps most dramatically off Nova Scotia, Canada, where 85-99% of kelp biomass (primarily Laminaria digitata and S. latissima) has been lost over the past 4-6 decades (Filbee-Dexter et al, 2016). Further south, steady warming of the NW Atlantic has been linked with decreased abundance of S. latissima near its equatorward range edge (Witman & Lamb, 2018;Feehan et al, 2019). Further south, steady warming of the NW Atlantic has been linked with decreased abundance of S. latissima near its equatorward range edge (Witman & Lamb, 2018;Feehan et al, 2019).…”

“…However, temperature-related biogeographic range shifts are less clear across the wider NW Atlantic, perhaps due to a lack of monitoring data (Merzouk & Johnson, 2011). increased grazing, space preemption by turf algae, or fouling by invertebrates) to accelerate kelp decline and suppress recovery (Filbee-Dexter et al, 2016;Witman & Lamb, 2018;Feehan et al, 2019). In many cases, the direct physiological effects of increased temperature were compounded by interacting ecological factors (e.g.…”

Projected 21st‐century distribution of canopy‐forming seaweeds in the Northwest Atlantic with climate change

“…Currently, the Gulf of Maine and Atlantic coast of Nova Scotia are already hot spots for climate change, with greater SST increases than other ocean regions (Pershing et al, ) leading to decreases in kelp abundance (Filbee‐Dexter et al, ; Krumhansl et al, ; Witman & Lamb, ), shifts in fucoid composition (Ugarte et al, ) and commercial fish stock collapses (Pershing et al, ). The current commercial rockweed harvest in Gulf of Maine and Canadian Maritimes (Seeley & Schlesinger, ) may not be sustained with continued warming as our projections suggest a shift from the dominance of A. nodosum with lower tolerance to warmer SST to the more opportunistic F. vesiculosus (Wilson et al, ).…”

“…Marine range shifts are more predictable (Sunday, Bates, & Dulvy, ) and occur at faster rates than terrestrial ones (Parmesan & Yohe, ; Sorte, Williams, & Carlton, ). Marine range shifts will affect habitat‐forming species in coastal habitats, with compounding impacts on the species who depend on their ecosystem structure, functions and services (Wernberg et al, ; Witman & Lamb, ). Significant losses of habitat‐forming species have already occurred in coral reefs (Carpenter et al, ), mangrove forests (Polidoro et al, ), seagrass meadows (Short et al, ) and seaweed beds (Krumhansl et al, ) due to multiple factors, and climate change poses an additional threat to many of these foundation species.…”