Post-glacial redistribution and shifts in productivity of giant kelp forests

Abstract

Quaternary glacial-interglacial cycles create lasting biogeographic, demographic and genetic effects on ecosystems, yet the ecological effects of ice ages on benthic marine communities are unknown. We analysed long-term datasets to develop a niche-based model of southern Californian giant kelp (Macrocystis pyrifera) forest distribution as a function of oceanography and geomorphology, and synthesized palaeooceanographic records to show that late Quaternary climate change probably drove high millennial variability in the distribution and productivity of this foundation species. Our predictions suggest that kelp forest biomass increased up to threefold from the glacial maximum to the mid-Holocene, then rapidly declined by 40 -70 per cent to present levels. The peak in kelp forest productivity would have coincided with the earliest coastal archaeological sites in the New World. Similar late Quaternary changes in kelp forest distribution and productivity probably occurred in coastal upwelling systems along active continental margins worldwide, which would have resulted in complex shifts in the relative productivity of terrestrial and marine components of coastal ecosystems.

Phylogeography of the supralittoral isopodLigia occidentalisaround the Point Conception marine biogeographical boundary

“…The presence of Clade A populations on the Channel Islands and the absence of Clade BCDE populations north of PC is consistent with the asymmetrical southward migration pattern around PC reported for several pelagically dispersing intertidal invertebrates (Wares et al, 2001;Hohenlohe, 2004;Cassone & Boulding, 2006). This is suggested by the high sequence divergence of the Santa Barbara Island population, as well as the apparently long-standing isolation of this small island from other landmasses (Graham et al, 2010). In contrast, the colonization of Santa Barbara Island by a member of Clade A probably represents a relatively old event that occurred via rafting.…”

“…The California coast has had a dynamic history, during which SST, ocean currents, sea level and the distribution of rocky intertidal habitat have changed dramatically over short evolutionary periods, affecting the distributions of marine species (Graham et al, 2010). from fossils or vicariant events) and knowledge of substitution rates in the mitochondrial genome of this isopod precludes the estimation of divergence times of L. occidentalis lineages.…”

Using non-dietary gastropods in coastal shell middens to infer kelp and seagrass harvesting and paleoenvironmental conditions

“…Interestingly, these data coincide with reconstructions of post-glacial shifts in giant kelp forests off the coast of mainland California, which predict increased kelp forest biomass during the Terminal Pleistocene and Early Holocene before declining rapidly as sea surface temperatures rose in the Middle Holocene (Graham et al 2010). Kelp forests surrounding the Channel Islands were proposed to have reached peak productivity during the Early Holocene followed by a significant decrease (50-80%) in kelp forest productivity and associated resources by the later part of the Middle Holocene as climatic conditions warmed (Graham et al 2010:404).…”

“…Decreasing trends in density could be reflective of a number of factors pertaining to human decision making and/or the environment, including variables such as changes in human harvesting practices and foraging locations, shifts in site function and settlement, regional or local environmental fluctuations, or decreasing kelp forest biomass as proposed by Graham et al (2010), which in turn affects species recruitment, settlement, and productivity. Our primary goal for this paper, however, is to highlight the use of marine macrophytes as inferred by epifauna in archaeological contexts and to provide interpretive possibilities for future studies.…”

Ecological niche models of invasive seaweeds

“…) and the historical extent of kelp beds in California (Graham et al. ). Although physiological responses to environmental variables have been studied for several notorious invasive species (e.g., Hanisak , Steen ), this has not resulted in mechanistic models to map habitat suitability and potential spread yet.…”