David G. Ainley

Marine Ecosystem Sensitivity to Climate Change

393 M ounting evidence suggests that the earth is experiencing a period of rapid climate change. Never before has it been so important to understand how environmental change influences the earth’s biota and to distinguish anthropogenic change from natural variability. Long-term studies in the western Antarctic Peninsula (WAP) region provide the opportunity to observe how changes in the physical environment are related to changes in the marine ecosystem. Analyses of paleoc limate records (MosleyThompson 1992, Peel 1992, Domack et al. 1993, Thompson et al. 1994, Dai et al. 1995, Domack and McClennen 1996, Leventer et al. 1996) have shown that the WAP region has moved from a relatively cold regime between approximately 2700 BP and 100 BP, to a relatively warm regime during the current century. Air temperature records from the last half-century show a dramatic warming trend, confirming the rapidity of change in the WAP area (Sansom 1989, Stark 1994, Rott et al. 1996, Smith et al. 1996). Significantly, polar ecosystem research over the last few decades (Fraser et al. 1992, Trivelpiece and Fraser 1996) and paleoecological records for the past 500 years (Emslie 1995, Emslie et al. 1998) reveal ecological transitions that have occurred in response to this climate change. In this article, we summarize the available data on climate variability and trends in the WAP region and discuss these data in the context of long-term climate variability during the last 8000 years of the Holocene. We then compare the available data on ecosystem change in the WAP region to the data on climate variability. Both historical and paleoenvironmental records indicate a climate gradient along the WAP that includes a dry, cold continental regime to the south and a wet, warm maritime regime to the north. The position of this climate gradient has shifted over time in response to the dominant climate regime, and it makes the WAP region a highly sensitive location for assessing ecological responses to climate variability. Our findings show that this century’s rapid climate warming has occurred concurrently with a shift in the population size and distribution of penguin species.

Increases in Antarctic penguin populations: reduced competition with whales or a loss of sea ice due to environmental warming?

SummaryA central tenet of Antarctic ecology suggests that increases in Chinstrap Penguin (Pygoscelis antarctica) populations during the last four decades resulted from an increase in prey availability brought on by the decrease in baleen whale stocks. We question this tenet and present evidence to support the hypothesis that these increases are due to a gradual decrease in the frequency of cold years with extensive winter sea ice cover resulting from environmental warming. Supporting data were derived from one of the first, major multidisciplinary winter expedition to the Scotia and Weddell seas; recent satellite images of ocean ice cover; and the analysis of long-term surface temperature records and penguin demography. Our observations indicate there is a need to pay close attention to environmental data in the management of Southern Ocean resources given the complexity of relating biological changes to ecological perturbations.

Breeding Performance in the Western Gull: Effects of Parental Age, Timing of Breeding and Year in Relation to Food Availability

(1) We studied age-specific breeding success of western gulls (Larus occidentalis) on south-east Farallon Island, California in relation to annual variation in food abundance, 1983-89. We assessed yearly reproductive performance on the basis of clutch initiation date, clutch size, hatching and fledging success, and the number of chicks raised to independence. We categorized age of parent into three groups: young, middle-aged and old. (2) Mature birds initiated breeding earlier and laid larger clutches; they fledged more chicks than younger birds as a result of hatching a greater proportion of their eggs and fledging a greater proportion of their hatched chicks. The oldest birds generally did not differ from middle-aged in breeding performance. The difference in reproductive performance between young and mature birds was most apparent during years when food availability was reduced. (3) Timing of breeding influenced reproductive performance independently of parental age; reproductive performance decreased throughout the breeding season. Within-season reductions in reproductive performance were most apparent for young breeders and individuals with small clutches or broods. (4) Breeding performance varied significantly among years independently of age of the study animals. Food availability accounted for part of the yearly variation in breeding success. (5) We suggest that clutch/brood size reflects individual quality. Age-specific breeding success in western gulls is best explained by previous breeding experience and individual quality.

Incidence of plastic in seabirds from the tropical pacific, 1984–1991: Relation with distribution of species, sex, age, season, year and body weight

Abstract The incidence of plastic in seabirds was studied (number of individuals of a species containing plastic per number inspected, and number of particles per individual), in 1574 individuals representing 36 species of seabirds collected in the tropical Pacific, mostly between 110 and 150 ° W longitude, from 1984 to 1991. Incidence of plastic was lower in resident species compared to those which bred to the south or north but wintered in the region, and especially when compared to species that crossed the tropics in migration between the South and North Pacific. Seasonal and age-related patterns in incidence of plastic, number of particles, and particle type (pellets versus user-plastic) among a group of five Procellariiform species (each with >5% of the individuals containing plastic and for which samples were >20 birds) indicated that degradation for an individual particle in the gizzard required less than one year, and that little plastic was regurgitated by parents to chicks. Two patterns emerged from this data regarding body weight: (i) heavier birds (for a given species, age-class, season and year) were more likely to contain at least some plastic, from which we hypothesize that birds in better physical condition fed more often in areas where higher densities of plastic and food are found, such as fronts and convergences; and (ii) among individuals who contained plastic (grouped by species), there was a significant negative correlation between number of plastic particles and body weight. This is the first solid evidence for a negative relationship between plastic ingestion and physical condition in seabirds. The likelihood that higher quality individuals are more prone to ingestplastic has serious implications regarding health of some seabird populations.

Ecological impact of a large Antarctic iceberg

[1] Satellite imagery has been used to document for the first time the potential for large icebergs to substantially alter the dynamics of a marine ecosystem. The B-15 iceberg (∼10,000 km 2 ), which calved off the Ross Ice Shelf in the biologically productive southwestern Ross Sea, Antarctica, restricted the normal drift of pack ice, resulting in heavier spring/summer pack ice cover than previously recorded. Extensive ice cover reduced both the area suitable for phytoplankton growth and the length of the algal growing season. Consequently, primary productivity throughout the region was >40% below normal, which changed both the abundance and behavior of upper trophic level organisms.

Sea-bird affinities for ocean and ice boundaries in the Antarctic

Abstract We have observed more sea birds near the Ross Sea continental slope and near ice than over the adjacent continental shelf, deep ocean, and ice-free regions. Bird concentrations near the continental margin occurred above th subsurface Antarctic Slope Front, defined by increased horizontal gradients in water temperature, salinity, density, chemistry, color, and transparency. A convergence between easterly and westerly currents near the continental shelf break may accumulate plankton and other positively bouyant material from oceanic gyres to the north and south. In addition, some sea-bird prey are believed to have life cycles adapted to the continental margin regime of upwelling deep water and sinking shelf water. Order-of-magnitude changes in bird abundance over relatively small distances indicate the importance of hydrographic and cryographic features to sea-bird distributions in the Southern Ocean.

Trophic interactions within the Ross Sea continental shelf ecosystem

The continental shelf of the Ross Sea is one of the Antarctics most intensively studied regions. We review the available data on the regions physical characteristics (currents and ice concentrations) and their spatial variations, as well as components of the neritic food web, including lower and middle levels (phytoplankton, zooplankton, krill, fishes), the upper trophic levels (seals, penguins, pelagic birds, whales) and benthic fauna. A hypothetical food web is presented. Biotic interactions, such as the role of Euphausia crystallorophias and Pleuragramma antarcticum as grazers of lower levels and food for higher trophic levels, are suggested as being critical. The neritic food web contrasts dramatically with others in the Antarctic that appear to be structured around the keystone species Euphausia superba. Similarly, we suggest that benthic–pelagic coupling is stronger in the Ross Sea than in most other Antarctic regions. We also highlight many of the unknowns within the food web, and discuss the impacts of a changing Ross Sea habitat on the ecosystem.

Diet and foraging effort of Adélie penguins in relation to pack-ice conditions in the southern Ross Sea

We investigated the diet and aspects of foraging effort among Adélie penguins (Pygoscelis adeliae) breeding at three colonies on Ross Island, in the southwestern Ross Sea – Capes Royds, Bird and Crozier – during the chick-provisioning period of three austral summers, 1994–1995, 1995–1996 and 1996–1997. During the study period, pack-ice cover differed in waters offshore of these colonies, by colony, seasons and year. Diet differed among colonies only slightly. The fish Pleuragramma antarcticum was the most important prey, especially during years or periods within years when little pack ice was present. With respect to krill, which composed the remainder of diet, juvenile Euphausia crystallorophias were consumed predominantly in a year of heavy pack-ice cover; more adult krill were consumed in 2 years when pack ice was sparse. Foraging trip duration differed by colony, season and year and was related directly to distance from the colony to the nearest pack ice. The amount of food brought to chicks increased as trip duration increased, to a point (2 days), but then decreased as duration increased further (up to 4 days). On the basis of data on mass of parents and of meal sizes to chicks, it appeared that on the longest trips more of the food gathered by parents was used for self maintenance; on the longest trips, parents lost body mass. Successful foraging during chick rearing, the period when adult foraging is most intense, appears to depend on the proximity of pack ice to nesting colonies for this penguin species.

GEOGRAPHIC STRUCTURE OF ADÉLIE PENGUIN POPULATIONS: OVERLAP IN COLONY‐SPECIFIC FORAGING AREAS

In an investigation of the factors leading to geographic structuring among Adelie Penguin (Pygoscelis adeliae) populations, we studied the size and overlap of colony- specific foraging areas within an isolated cluster of colonies. The study area, in the south- western Ross Sea, included one large and three smaller colonies, ranging in size from 3900 to 135 000 nesting pairs, clustered on Ross and Beaufort Islands. We used triangulation of radio signals from transmitters attached to breeding penguins to determine foraging loca- tions and to define colony-specific foraging areas during the chick-provisioning period of four breeding seasons, 1997-2000. Colony populations (nesting pairs) were determined using aerial photography just after egg-laying; reproductive success was estimated by com- paring ground counts of chicks fledged to the number of breeding pairs apparent in aerial photos. Foraging-trip duration, meal size, and adult body mass were estimated using RFID (radio frequency identification) tags and an automated reader and weighbridge. Chick growth was assessed by weekly weighing. We related the following variables to colony size: foraging distance, area, and duration; reproductive success; chick meal size and growth rate; and seasonal variation in adult body mass. We found that penguins foraged closest to their respective colonies, particularly at the smaller colonies. However, as the season pro- gressed, foraging distance, duration, and area increased noticeably, especially at the largest colony. The foraging areas of the smaller colonies overlapped broadly, but very little foraging area overlap existed between the large colony and the smaller colonies, even though the foraging area of the large colony was well within range of the smaller colonies. Instead, the foraging areas of the smaller colonies shifted as that of the large colony grew. Colony size was not related to chick meal size, chick growth, or parental body mass. This differed from the year previous to the study, when foraging trips of the large colony were very long, parents lost mass, and chick meals were smaller. In light of existing data on prey abundance in neritic waters in Antarctica suggesting that krill are relatively evenly distributed and in high abundance in the Southern Ross Sea, we conclude that penguins depleted or changed the availability of their prey, that the degree of alteration was a function of colony size, and that the large colony affected the location (and perhaps ultimately the size) of foraging areas for the smaller colonies. It appears, therefore, that foraging dynamics play a role in the geographic structuring of colonies in this species.

SPATIAL AND TEMPORAL VARIATION OF DIET WITHIN A PRESUMED METAPOPULATION OF ADÉLIE PENGUINS

Abstract We investigated temporal and spatial variability in the diet of chick-provisioning Adélie Penguins (Pygoscelis adeliae) breeding at all colonies within one isolated cluster in the southwestern Ross Sea, Antarctica, 1994–2000. We wished to determine if prey quality explained different population growth and emigration rates among colonies. Diet composition was described both by conventional means (stomach samples) and by analysis of stable isotopes in chick tissues (toenails of individuals killed by skuas [Stercorarius maccormicki]). Diets were similar among the four study colonies compared to the disparity apparent among 14 widely spaced sites around the continent. Calorimetry indicated that fish are more energetically valuable than krill, implying that if diet varied by colony, diet quality could attract recruits and help to explain differential rates of colony growth. However, a multiple-regression analysis indicated that diet varied as a function of year, time within the year, and percent of foraging area covered by sea ice, but not by colony location. Stable isotopes revealed similarity of diet at one colony where conventional sampling was not possible. We confirmed that sea ice importantly affects diet composition of this species in neritic waters, and found that (1) quality of summer diet cannot explain different population growth rates among colonies, and (2) stable isotope analysis of chick tissues (toenails) is a useful tool to synoptically describe diet in this species over a large area. Variación Espacial y Temporal de la Dieta en una Supuesta Metapoblación de Pygoscelis adeliae Resumen. Investigamos la variabilidad temporal y espacial en la dieta de Pygoscelis adeliae que se encontraban aprovisionando a sus polluelos en todas las colonias dentro de un grupo aislado en el mar de Ross sud-occidental, Antártica, entre 1994 y 2000. Deseábamos determinar si la calidad de la presa explicaba las diferentes tasas de crecimiento poblacional y emigración entre colonias. La composición de la dieta fue descrita por medios convencionales (muestras estomacales) y por el análisis de isótopos estables en tejidos de los polluelos (uñas de las patas de los individuos matados por Stercorarius maccormicki). Las dietas fueron similares entre las 4 colonias estudiadas en relación a la disparidad de la dieta evidente entre 14 sitios dispersos a través del continente. Las análisis de calorimetría indicaron que los peces tienen un mayor valor energético que el krill, sugiriendo que si la dieta varía entre colonias, la calidad de la dieta podría atraer a reclutas y ayudaría a explicar los diferentes índices de crecimiento entre colonias. Sin embargo, un análisis de regresión múltiple indicó que la dieta varió en función del año, de la época dentro de un año, y del porcentaje de área de forrajeo cubierta por el hielo del mar, pero no en función de la localización de la colonia. Los isótopos estables revelaron la semejanza de la dieta en una colonia en que no era posible el muestreo convencional. Confirmamos que el hielo en el mar afecta de manera importante la composición de la dieta de esta especie en aguas neríticas, y encontramos que (1) la calidad de la dieta de verano no puede explicar las diferentes tasas de crecimiento poblacional entre colonias, y que (2) el análisis de isótopos estables de los tejidos de polluelos (uñas de las patas) es una herramienta útil para describir, sinópticamente, la dieta en esta especie a través de un área extensa.