Andrew J. Pershing

ARCTIC CLIMATE CHANGE AND ITS IMPACTS ON THE ECOLOGY OF THE NORTH ATLANTIC

Arctic climate change from the Paleocene epoch to the present is reconstructed with the objective of assessing its recent and future impacts on the ecology of the North Atlantic. A recurring theme in Earths paleoclimate record is the importance of the Arctic atmosphere, ocean, and cryosphere in regulating global climate on a variety of spatial and temporal scales. A second recurring theme in this record is the importance of freshwater export from the Arctic in regulating global- to basin-scale ocean circulation patterns and climate. Since the 1970s, historically unprecedented changes have been observed in the Arctic as climate warming has increased precipitation, river discharge, and glacial as well as sea-ice melting. In addition, modal shifts in the atmosphere have altered Arctic Ocean circulation patterns and the export of freshwater into the North Atlantic. The combination of these processes has resulted in variable patterns of freshwater export from the Arctic Ocean and the emergence of salinity anomalies that have periodically freshened waters in the North Atlantic. Since the early 1990s, changes in Arctic Ocean circulation patterns and freshwater export have been associated with two types of ecological responses in the North Atlantic. The first of these responses has been an ongoing series of biogeographic range expansions by boreal plankton, including renewal of the trans-Arctic exchanges of Pacific species with the Atlantic. The second response was a dramatic regime shift in the shelf ecosystems of the Northwest Atlantic that occurred during the early 1990s. This regime shift resulted from freshening and stratification of the shelf waters, which in turn could be linked to changes in the abundances and seasonal cycles of phytoplankton, zooplankton, and higher trophic-level consumer populations. It is predicted that the recently observed ecological responses to Arctic climate change in the North Atlantic will continue into the near future if current trends in sea ice, freshwater export, and surface ocean salinity continue. It is more difficult to predict ecological responses to abrupt climate change in the more distant future as tipping points in the Earths climate system are exceeded.

Climate and the conservation biology of North Atlantic right whales: the right whale at the wrong time?

With the end of commercial whaling, it was thought that populations of the highly endangered North Atlantic right whale (Eubalaena glacialis) would gradually recover. However, recent modeling studies have shown that the population’s growth rate increased gradually during the 1980s, but began declining in the early 1990s, when female mortality rates increased substantially. Demographic projections predict that, assuming birth and mortality rates remain comparable to those observed in the early 1990s, the population will become extinct in less than 200 years. Further extrapolations suggest that reducing mortality rates by a few female deaths per year through conservation efforts would be sufficient to support a slow recovery of the population. However, the effects of climate variability and change on calving rates may make the North Atlantic right whale even more vulnerable than previous projections have suggested. Failure to incorporate the effects of climate in demographic projections may lead us to under...

Assessing the distribution and abundance of zooplankton : a comparison of acoustic and net-sampling methods with D-BAD MOCNESS

Abstract Results are described from the first field study with the D-BAD MOCNESS (Dual-Beam Acoustics Deployed on a Multiple Opening/Closing Net and Environmental Sensing System), an instrument designed to collect acoustic data and net samples simultaneously from the same portion of the water column. Our primary objective was to evaluate the advantages and disadvantages of integrating in a single instrument these two very distinctive methods for assessing the distribution and abundance of zooplankton. In this context, we required a means of comparison that would enable us to groundtruth acoustic remote-sensing data with net sample data. The approach chosen, referred to as the forward-problem approach, compares the acoustic volume backscattering coefficients observed in situ with those predicted from net sample data and acoustic scattering models. The results from this study show that the observed acoustic volume backscattering data are generally consistent with the forward-problem predictions. This consistency is true in terms of both total acoustic volume backscattering as well as that portion of the volume backscattering contributed by each of the dominant sound scatterer types. The results also provide two examples of situations in which inconsistencies between the observed and predicted acoustic volume backscattering can be used to detect potential methodological problems. D-BAD MOCNESS appears to be a useful instrument for groundtruthing acoustic data; however, due to its slow towing speed, it is not a suitable instrument for large-scale, acoustic survey work.

Evidence for vertical circulation cells in the well-mixed area of Georges Bank and their biological implications

Two surveys were conducted in the well-mixed region of Georges Bank to look for secondary vertical circulation cells, the first in 1996 and the second in 1997. Each survey collected high-frequency acoustic, temperature, and fluorescence data along a 1-n.mile square grid. Concurrent ADCP measurements also were made in the second year. MOCNESS and pump samples from both years caught large amounts of sand and organisms typical of this regions such as copepods and hydroids. However, forward problem calculations suggest that the acoustic scattering was dominated by post-larval bivalves. Sand and copepods also accounted for significant amounts of the estimated backscatter. The acoustic data from both surveys contained near-surface vertical bands of high-volume backscatter. The frequency and intensity of these bands was strongly correlated with the magnitude of the current velocity. Significant upwelling and downwelling were observed in the ADCP records, and the acoustic bands often co-occurred in the downwelling zones. Simulations of particle distributions within idealized circulation cells, consistent with the acoustic and ADCP data, suggest that the acoustic bands are caused by aggregations of positively buoyant or upward-swimming scatterers. The circulation cells proposed could have an important effect on the ecology of the well-mixed region by aggregating upward-swimming fish and zooplankton in near-surface patches.