2009 2 A compilation of Late Pliensbachian and Early Toarcian marine biotic 3 changes 4 5

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Severe paleoclimatic change during the Toarcian (Early Jurassic) oceanic anoxic event (OAE) was characterized by a negative δ 13 C excursion, increased weathering, higher seawater temperatures, oceanic deoxygenation, and mass extinction. We present abundance and size data (n ≈ 36,000) for the two dominant epifaunal bivalve species from the Toarcian OAE, Yorkshire, UK. We statistically correlate the biotic data with geochemical proxies for environmental change and show that our results are comparable with changes in present-day ecosystems affected by hypoxia. Bositra radiata dominated during declining oxygen levels immediately before the Toarcian OAE sensu stricto, and shell size doubled when δ 13 C org was decreasing, indicating a connection with primary productivity. Small Pseudomytiloides dubius dominated during the Toarcian OAE and varied sharply in abundance, indicating that it was highly opportunistic. P. dubius shell size is strongly related to Mo concentration, [Mo]; this indicates a relationship between size and N 2 -fi xing primary productivity via [Mo] limitation. A secondary factor contributing to small shell size was lower oxygen levels. After the Toarcian OAE diversity increased, P. dubius was less abundant and shell size doubled, indicating that bivalve populations were less limited by resources and conditions were more favorable. Size frequency distributions show that bothToarcian species had short life spans, rapid generation times, high recruitment, and high juvenile mortality. The opportunist Mulinia lateralis is a present-day analog for P. dubius. This research provides a case study for the long-term impacts of deoxygenation upon marine ecosystems, including that being observed today.

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Global warming during the Early Jurassic, and associated widespread ocean deoxygenation, was comparable in scale with the changes projected for the next century. This study quantifies the impact of severe global environmental change on the biological traits of marine communities that define the ecological roles and functions they deliver. We document centennial-millennial variability in the biological trait composition of Early Jurassic (Toarcian) seafloor communities and examine how this changed during the event using biological traits analysis. Environmental changes preceding the global oceanic anoxic event (OAE) produced an ecological shift leading to stressed benthic palaeocommunities with reduced resilience to the subsequent OAE. Changes in traits and ecological succession coincided with major environmental changes; and were of similar nature and magnitude to those in severely deoxygenated benthic communities today despite the very different timescales. Changes in community composition were linked to local redox conditions whereas changes in populations of opportunists were driven by primary productivity. Throughout most of the OAE substitutions by tolerant taxa conserved the trait composition and hence functioning, but periods of severe deoxygenation caused benthic defaunation that would have resulted in functional collapse. Following the OAE recovery was slow probably because the global nature of the event restricted opportunities for recruitment from outside the basin. Our findings suggest that future systems undergoing deoxygenation may initially show functional resilience, but severe global deoxygenation will impact traits and ecosystem functioning and, by limiting the species pool, will slow recovery rates.

The prevalence of crustaceans, such as fiddler crabs, in tropical mudflats can provide functional equivalence to the burrowing worms of temperate regions.

Deoxygenation has profound eff ects on marine biota and delivery of ecological functions in benthic systems. Globally, coastal and oceanic hypoxia is rapidly increasing due to anthropogenic activities including climate change and eutrophication. Little is known about the response of marine ecosystems to deoxygenation over long timescales and the consequences this will have for functioning. Th is study presents results from biological traits analysis (BTA) of 21 time averaged benthic palaeocommunities from the Wessex Basin, Dorset, UK representing ~ 8 million years of fl uctuating regional hypoxia during the Kimmeridgian ( ∼ 148 -155 Ma). BTA assesses ecosystem functioning using biological traits expressed by species and has not previously been applied to palaeocommunity data.Th e preserved remains of the palaeocommunities contained gastropods, brachiopods, scaphopods, bryozoans, serpulids, hydroids and crustaceans, but were dominated by bivalves. Signifi cant changes in species composition are shown within periods of less intense hypoxia, but during these periods trait composition did not signifi cantly diff er implying conservation of ecological function. However, signifi cant changes in functioning occurred between periods of extremely diff erent palaeoredox state. Proportionally more surface living or shallow burrowing species with traits suggestive of opportunists occurred during periods of low oxygen availability. Morphological diff erences of hypoxic communities included higher relative abundance of organisms with thinner skeletons ( Ͻ 0.5 mm) composed of less soluble forms of calcite that may be linked to acidity. Th ese changes are similar to those for modern benthos exposed to hypoxia.Investigation of functional changes that occurred during ancient hypoxic events can be used to infer the magnitude, thresholds, and rates of long-term functional change in modern communities. Results from this study suggest that during de-oxygenation delivery of normal benthic functioning will initially be maintained, but will collapse once thresholds are reached. Th is is consistent with the patterns emerging for contemporary systems where functional collapse is associated with hysteresis and threshold eff ects.

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