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.
Highlights
IntroductionForests of Laminarian algae (kelp) provide many ecosystem services such as supporting nursery habitats, creating a productive, edible habitat for diverse assemblages of invertebrates and fish, contributing nutritive detritus for different groups of consumers, and dampening wave action [1, 2, 3]
Forests of Laminarian algae provide many ecosystem services such as supporting nursery habitats, creating a productive, edible habitat for diverse assemblages of invertebrates and fish, contributing nutritive detritus for different groups of consumers, and dampening wave action [1, 2, 3]
Temperature analysis indicated that the Gulf of Maine has warmed since the study began in 1987 (Fig 2)
Summary
Forests of Laminarian algae (kelp) provide many ecosystem services such as supporting nursery habitats, creating a productive, edible habitat for diverse assemblages of invertebrates and fish, contributing nutritive detritus for different groups of consumers, and dampening wave action [1, 2, 3]. Local drivers of kelp abundance include sea urchin grazing [2, 4, 5, 6], storms [1, 7, 8], pollution [9], invasive species impacts [10, 11, 12] and indirect effects of top predators [13], while global drivers are linked to the El Niño Southern Oscillation [14,15] and ocean warming as a manifestation of climate change [3, 16] By virtue of their relatively rapid rates of growth and recruitment, kelp are relatively resilient to disturbance [14, 17], significant declines in kelp have occurred in concert with ocean warming in some areas off Western Europe and Australia [16, 18]. This study pointed out that more research on patterns and drivers of kelp population change within regions is needed to resolve the potential large-scale erosion of ecosystem services provided by these important foundation species [19]
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