Bettina Meyer 10 Dec 2009
Thursday 10 December 2009, 11:30 AM
Bettina Meyer
Alfred-Wegener Institute (AWI), Bremerhaven, Germany
Overwintering of krill with emphasis on the adult stages
Recent studies suggest that the overwintering success of Euphausia superba, especially of the larval stages (the new recruits), is the major single factor that dictates their recruitment success and hence total population size. However, the potential overwintering mechanisms of krill, particularly of their larval stages, are still poorly understood. We still lack sufficient understanding of overwintering behaviour to predict how krill will respond to future climatic changes.
Adult krill have a suite of overwintering mechanisms that consist, on the one hand, of reduced metabolism, feeding activity and growth (and even shrinkage) plus utilization of lipid reserves and, on the other hand, switching to alternative food sources other than phytoplankton in the water column, namely ice algae, zooplankton and sea bed detritus. In contrast to adults, krill larvae have low lipid reserves and cannot tolerate long starvation periods, suggesting that adults and larvae have different mechanisms to survive the winter season. Previously, these overwintering mechanisms have been observed separately or as a combination of only a few parameters, mainly in the vicinity of the Antarctic Peninsula. Consequently, until the end of the 1990s, there was a great deal of speculation on krill overwintering, much of it supported by very little solid data. The prevailing view was that krill had a flexible approach to overwintering, with some ontogenetic differences, but beyond this the exact mechanisms were subject to vigorous debate.
To overcome these uncertainties, the aims of the working group Antarctic krill at the Alfred-Wegener Institute (AWI) since 1999 were to investigate:
a) to what extent seasonal variation in larval and adult krill physiology is mediated by environmental factors with strong seasonality in the Southern Ocean, such as food supply or daylight, and
b) which physiological functions are adopted by adults and larvae to survive the winter season and the relative importance of each.
In order to achieve these aims, parameters thought to be associated with proposed overwintering mechanisms (carbon-, nitrogen-, lipid-, protein content, metabolic rates, feeding activity and growth) were determined during different seasons with a consistent experimental and analytical setup to ensure a high degree of comparability. Such an approach enabled us to determine the relative importance of each component measured, and to make a rigorous comparison of the mechanisms adopted by larvae and adult krill. It also allowed the identification of environmental factors that drive the variation in the physiological function of krill throughout the seasonal cycle.
Field studies took place on research cruises with R/V "Polarstern"in the Lazarev Sea (ANTXVI-3, ANTXXI-4, ANTXXIII-2, ANTXXIII-6) and the Eastern Bellingshausen Sea (ANTXVIII-5b). Furthermore, a field study was performed in Marguerite Bay at the British Antarctic Survey station "Rothera" and laboratory experiments with larval and adult krill at the Australian Antarctic Division in Kingston, Tasmania.
Adult krill
Results on adult krill have demonstrated that specific physiological adaptations during autumn and winter, such as reduced metabolic rates and feeding activity, are not caused simply from the scarcity of food, as was previously assumed. They represent an inherent adaptational strategy that appears to be influenced by the local light regime. The seasonal lipid dynamics in adult krill consist of a clear annual pattern of accumulation of lipids during summer until the onset of winter and their utilisation during winter. Hence, it is most likely that this pattern is also an inherent strategy of krill that has evolved in anticipation of the succession of seasons.
The seasonal investigations have shown that metabolic depression during winter is an important and very efficient energy saving mechanism. They also revealed that the remaining loss due to respiration is not compensated by the usage of energy reserves, mainly lipids, alone. Thus, feeding activity, although very low, is an essential part to successfully survive the winter season. Opportunistic feeding and combustion of body stores in combination with a reduced metabolic activity ensure that the animals lose condition only slowly during the winter season. The low feeding- and metabolic activity is accompanied by low to zero growth and even body shrinkage. Based on our results and the available literature, body shrinkage does not seem to be a general overwintering strategy of adult krill. It is rather an alternative way for krill to conserve energy in extreme conditions, when stored lipid reserves are almost depleted.
Because of these inherent physiological adaptations of adult krill, sea ice appears to play only a minor role for adults during winter. With the onset of spring, when metabolic rates increase and lipid levels are low due to winter depletion, sea ice appears to become an important feeding ground for adult krill to accumulate lipid and gametes and undergo successful spawning.
Larval krill
The physiological functions of larval krill adopted during winter (reduced metabolism, delayed development, lipid utilization) are, in contrast to the adults, due to the behaviour under stronger direct control by the available food supply. In all investigated seasons (summer, autumn and winter) larvae showed a positive functional response in metabolic and feeding activity with increasing food availability, suggesting that no factor other than food is involved in the seasonal variability in physiological function.
The growth rates of larval krill are highly variable from the onset of winter in April until the end in September. The larvae display a clear positive growth in April, which decreases steadily to a minimum from June to August and increases again in September. This pattern reflects the high variability in food supply from late autumn to the end of winter. Larval krill also showed a large flexibility in the frequency and mode of growth: The duration of the intermoult periods during winter is nearly double that recorded during summer and autumn. Furthermore, the larvae follow direct and indirect developmental pathways. During direct development, larvae moult to the next ontogenetic stage, whereas they moult to the same stage (delayed development) or to an intermediate form during indirect development. In mid winter, the majority of FVI larvae moulted to the same stage, whereas FIV and FV moulted to an intermediate form when an indirect developmental pathway was observed. Similar to adults, body shrinkage is not a general overwintering strategy but rather a flexible behaviour to overcome severe condition such as low lipid levels resulting from low food supply.
Autotrophic food plays an important role during summer and autumn, whereas heterotrophic food becomes increasingly important during winter. Larvae have to feed during winter to meet their energetic needs since their lipid levels are too low to sustain several winter months without food. The lipid levels increase with ontogenetic stage from CI to FVI, and, consequently, their tolerance to short starvation periods extends from days to weeks. The late larval stages (FIV-FVI) from early-spawning krill therefore have a higher chance of surviving periods of food depletion during winter than younger larvae deriving from late-spawning krill.
These results all demonstrate that larvae have a flexible behaviour to overcome short term starvation periods but they are unable to survive long periods of food shortage, in contrast to adults. Therefore, a problematic point is that, on one hand, larvae have to feed continuously to meet their energetic demands but, on the other hand, they live in an environment of frequent high current speed, which might preclude each other. Therefore, irregular sea ice topography may be essential for the larvae not only as a feeding- but also as resting ground, allowing them to remain in favourable ice habitat. The interplay between under-ice topography, apparent current speed under sea ice and the swimming ability of larval krill is probably critical as to whether larval krill can maintain position and exploit suitable feeding areas under the ice.
In conclusion, our perception of krill overwintering has undergone a fundamental change in the last 10 years. The long-standing paradigm has been that ice algae provide an important winter food source for krill. Our findings, and others, suggest that ice algae are not the major food, either for adults or larvae. While the adults often seem to have little association with ice (at least until early spring), the larvae feed within sea ice, but mainly on the grazers of the ice-algal community rather than the algae themselves.
Bio - Dr. Bettina Meyer is head of the working group Antarctic Krill at the Alfred-Wegener Institute (AWI) in Bremerhaven Germany (since 2000). The focus of this working group is to understand the population dynamics of krill from a physiological view point, with emphasis on the question of how larval and adult krill overwinter. Since 2005 she has collaborated closely with the krill group at the AAD. The laboratory experiments already performed and in progress focus on the influence of light on krill physiological functions and underlying mediating mechanisms. Bettina is currently a visiting scientist at the AAD (7th Nov to 17th Dec 2009) to initiate a one year duration light experiment aimed to understand if krill has an internal clock and if so how this functions.
