The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition. 
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet. 
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources. 
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.

Influence of Diet On the Distribtion of Nitrogen Isotopes in Animals

Marine ecosystems are centrally important to the biology of the planet, yet a comprehensive understanding of how anthropogenic climate change is affecting them has been poorly developed. Recent studies indicate that rapidly rising greenhouse gas concentrations are driving ocean systems toward conditions not seen for millions of years, with an associated risk of fundamental and irreversible ecological transformation. The impacts of anthropogenic climate change so far include decreased ocean productivity, altered food web dynamics, reduced abundance of habitat-forming species, shifting species distributions, and a greater incidence of disease. Although there is considerable uncertainty about the spatial and temporal details, climate change is clearly and fundamentally altering ocean ecosystems. Further change will continue to create enormous challenges and costs for societies worldwide, particularly those in developing countries.

http://science.sciencemag.org/content/sci/328/5985/1523.full.pdf

The impact of climate change on the world's marine ecosystems.

The El Nino-Southern Oscillation (ENSO) cycle of alternating warm El Nino and cold La Nina events is the dominant year-to-year climate signal on Earth. ENSO originates in the tropical Pacific through interactions between the ocean and the atmosphere, but its environmental and socioeconomic impacts are felt worldwide. Spurred on by the powerful 1997-1998 El Nino, efforts to understand the causes and consequences of ENSO have greatly expanded in the past few years. These efforts reveal the breadth of ENSO's influence on the Earth system and the potential to exploit its predictability for societal benefit. However, many intertwined issues regarding ENSO dynamics, impacts, forecasting, and applications remain unresolved. Research to address these issues will not only lead to progress across a broad range of scientific disciplines but also provide an opportunity to educate the public and policy makers about the importance of climate variability and change in the modern world.

/pdf/enso-as-an-integrating-concept-in-earth-science-52h9llx7lf.pdf

ENSO as an Integrating Concept in Earth Science

The Intergovernmental Panel on Climate Change (IPCC) confirmed that mean global warming was 0.6 ± 0.2 °C during the 20th century and cited anthropogenic increases in greenhouse gases as the likely cause of temperature rise in the last 50 years. But this mean value conceals the substantial complexity of observed climate change, which is seasonally- and diurnally-biased, decadally-variable and geographically patchy. In particular, over the last 50 years three high-latitude areas have undergone recent rapid regional (RRR) warming, which was substantially more rapid than the global mean. However, each RRR warming occupies a different climatic regime and may have an entirely different underlying cause. We discuss the significance of RRR warming in one area, the Antarctic Peninsula. Here warming was much more rapid than in the rest of Antarctica where it was not significantly different to the global mean. We highlight climate proxies that appear to show that RRR warming on the Antarctic Peninsula is unprecedented over the last two millennia, and so unlikely to be a natural mode of variability. So while the station records do not indicate a ubiquitous polar amplification of global warming, the RRR warming on the Antarctic Peninsula might be a regional amplification of such warming. This, however, remains unproven since we cannot yet be sure what mechanism leads to such an amplification. We discuss several possible candidate mechanisms: changing oceanographic or changing atmospheric circulation, or a regional air-sea-ice feedback amplifying greenhouse warming. We can show that atmospheric warming and reduction in sea-ice duration coincide in a small area on the west of the Antarctic Peninsula, but here we cannot yet distinguish cause and effect. Thus for the present we cannot determine which process is the probable cause of RRR warming on the Antarctic Peninsula and until the mechanism initiating and sustaining the RRR warming is understood, and is convincingly reproduced in climate models, we lack a sound basis for predicting climate change in this region over the coming century.

Recent Rapid Regional Climate Warming on the Antarctic Peninsula

Antarctic krill (Euphausia superba) and salps (mainly Salpa thompsoni) are major grazers in the Southern Ocean, and krill support commercial fisheries. Their density distributions have been described in the period 1926-51, while recent localized studies suggest short-term changes. To examine spatial and temporal changes over larger scales, we have combined all available scientific net sampling data from 1926 to 2003. This database shows that the productive southwest Atlantic sector contains >50% of Southern Ocean krill stocks, but here their density has declined since the 1970s. Spatially, within their habitat, summer krill density correlates positively with chlorophyll concentrations. Temporally, within the southwest Atlantic, summer krill densities correlate positively with sea-ice extent the previous winter. Summer food and the extent of winter sea ice are thus key factors in the high krill densities observed in the southwest Atlantic Ocean. Krill need the summer phytoplankton blooms of this sector, where winters of extensive sea ice mean plentiful winter food from ice algae, promoting larval recruitment and replenishing the stock. Salps, by contrast, occupy the extensive lower-productivity regions of the Southern Ocean and tolerate warmer water than krill. As krill densities decreased last century, salps appear to have increased in the southern part of their range. These changes have had profound effects within the Southern Ocean food web.

Long-term decline in krill stock and increase in salps within the Southern Ocean.

Surveys of Euphausia superba often target localised shelves and ice edges where their growth rates and predation losses are atypically high. Emphasis on these areas has led to the current view that krill require high food concentrations, with a distribution often linked to shelves. For a wider, circumpolar perspective, we compiled all available net-based density data on postlarvae from 8137 mainly summer stations from 1926 to 2004. Unlike Antarctic zooplankton, the distribution of E. superba is highly uneven, with 70% of the total stock concentrated between longitudes 0° and 90°W. Within this Atlantic sector, krill are abundant over both continental shelf and ocean. At the Antarctic Peninsula they are found mainly over the inner shelf, whereas in the Indian–Pacific sectors krill prevail in the ocean within 200 to 300 km of the shelf break. Overall, 87% of the total stock lives over deep oceanic water (>2000 m), and krill occupy regions with moderate food concentrations (0.5 to 1.0 mg chl a m–3). Advection models suggest some northwards loss from these regions and into the low chlorophyll belts of the Antarctic Circumpolar Current (ACC). We found possible evidence for a compensating southwards migration, with an increasing proportion of krill found south of the ACC as the season progresses. The retention of krill in moderately productive oceanic habitats is a key factor in their high total production. While growth rates are lower than over shelves, the ocean provides a refuge from shelf-based predators. The unusual circumpolar distribution of krill thus reflects a balance between advection, migration, top–down and bottom–up processes.

/pdf/oceanic-circumpolar-habitats-of-antarctic-krill-15chr5zu0m.pdf

Oceanic circumpolar habitats of Antarctic krill

/pdf/foundations-for-ecological-research-west-of-the-antarctic-4vj2wc6e12.pdf

Foundations for Ecological Research West of the Antarctic Peninsula

A unique Antarctic data set from four multidisciplinary cruises (spring, Nov 1991; summer, Jan–Feb 1993; fall, Mar–May 1993; winter, Aug–Sept 1993) was analyzed to provide a description of seasonal variability in the distribution and abundance of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula. Analyses of acoustic observations revealed distinct seasonal variations in estimates of krill abundance, the dimensional parameters used to characterize individual krill aggregations, the geographic area over which krill were acoustically detected, and the depth distribution of krill biomass. Spatially averaged estimates of krill biomass were an order of magnitude higher in spring (32 g m-2) and summer (95 g m-2) compared to fall (12 g m-2) and winter (8 g m-2). In summer, krill were detected throughout the region and most of the krill biomass was associated with aggregations of small cross-sectional area ( 150 g m-3), which were positioned in the upper 50 m of the water column. Winter observations, in contrast, were characterized by the absence of krill throughout most of the region and aggregations occuring deeper than 100 m with large cross-sectional area (>10000 m2) and low mean biomass ( 50 g m-2) at selected locations on the inner shelf and the disproportionate contribution, in terms of total krill biomass (>80%), by a small number of aggregations ( 40 mm) krill positioned offshore of smaller krill (32–38 mm) in all seasons except winter. Synthesis of these data and historical observations suggest that there is a seasonal shift in the primary habitat of krill and that changes in krill behavior are an important factor affecting variations in krill distributions.

Seasonal variability in the distribution of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula

This paper describes spatial distribution patterns of the phytoplankton community (com- position, cell abundance and biomass concentration) in relation to local environmental conditions in the Southern Ocean. Sampling was performed during summer 1997 off the coast of the western Antarctic Peninsula between Anvers Island and Marguerite Bay. Phytoplankton was characterized by relatively low biomass throughout most of the study area and was dominated by nanoalgae (<20 µm). Phyto- plankton varied along an on-offshore gradient, with decreasing total cell abundance, chlorophyll a (chl a) concentration and carbon biomass toward the open ocean. Chl a concentration showed surface or subsurface maxima in coastal and middle-shelf waters, and deep maxima between ~40 and 100 m in oceanic waters. Across-shelf variability in phytoplankton correlated with vertical stability in the water column, which appears to be the major parameter affecting phytoplankton community structure in the area. We hypothesize that the deep chl a maximum offshore may be associated with iron limitation in near-surface waters and higher iron concentration in 'winter waters' (subsurface remnant of Antarctic Surface Waters). On a smaller spatial scale, a cluster analysis showed great regional variability in phyto- plankton assemblages. The area was divided into 4 main regions based on differences in the phyto- plankton composition and concentration. Three peaks in phytoplankton abundance were found on a north-to-south gradient in near-shore waters: a Cryptomonas spp. bloom near Anvers Island, a small unidentified phytoflagellate bloom in Grandidier Channel, and a diatom bloom in Marguerite Bay. These assemblages resemble different stages of the phytoplankton seasonal succession, and may be related to the progressive sea-ice retreat, which might have regulated the timing of the onset of the phytoplank- ton seasonal succession in a north-south gradient. Biological environmental factors, such as seeding of the water column by epontic algae and selective zooplankton herbivory, are hypothesized to affect com- munity composition in coastal regions. We conclude that large-scale variability in phytoplankton com- munity structure is related to water column physical conditions and possibly iron availability, while mesoscale variability, as seen in coastal waters, is more likely due to seasonal succession of different algae groups.

/pdf/phytoplankton-spatial-distribution-patterns-along-the-4h1sdhdidn.pdf

Phytoplankton spatial distribution patterns along the western Antarctic Peninsula (Southern Ocean)

As part of the Palmer Long-Term Ecological Research (LTER) program, the size struc- ture of the population of Antarctic krill found between Anvers and Adelaide Islands west of the Antarctic Peninsula has been analyzed for patterns in recruitment success. The data were from a series of 11 cruises, 1 in spring (November 1991) and 10 in summer (January 1993 to January 2002). A maximum-likelihood fitting procedure was used to fit a mixture of normal distributions to the length-density distributions derived from net data. The recruitment index was calculated as the pro- portion of age-class 1 krill of the total. Antarctic krill in the Palmer LTER region showed a pattern of episodic recruitment, with 2 strong year classes in succession followed by 3 or 4 moderate or poor year classes. The 2 strong year classes represented 85 to 90% of the krill caught for 5 or 6 yr, and as the absolute abundance of the year class declined due to mortality, so did the abundance of Antarc- tic krill in the region. The recruitment index was positively correlated with the absolute value of a seasonal El Nino/Southern Oscillation (ENSO) index, with strongest recruitment during the neutral or moderate periods of ENSO. The mechanism underlying the strong link between the recruitment index and ENSO is most likely the effects of seasonal sea-ice dynamics on both reproduction and winter-over survival of the resulting larvae as previously documented.

/pdf/episodic-recruitment-in-antarctic-krill-euphausia-superba-in-3c06mgcko3.pdf

Robin M. Ross

Papers

Oceanic circumpolar habitats of Antarctic krill

Foundations for Ecological Research West of the Antarctic Peninsula

Seasonal variability in the distribution of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula

Phytoplankton spatial distribution patterns along the western Antarctic Peninsula (Southern Ocean)

Episodic recruitment in Antarctic krill Euphausia superba in the Palmer LTER study region