Andrew G. Hirst

Spatial and temporal operation of the Scotia Sea ecosystem: a review of large-scale links in a krill centred food web

The Scotia Sea ecosystem is a major component of the circumpolar Southern Ocean system, where productivity and predator demand for prey are high. The eastward-flowing Antarctic Circumpolar Current (ACC) and waters from the Weddell–Scotia Confluence dominate the physics of the Scotia Sea, leading to a strong advective flow, intense eddy activity and mixing. There is also strong seasonality, manifest by the changing irradiance and sea ice cover, which leads to shorter summers in the south. Summer phytoplankton blooms, which at times can cover an area of more than 0.5 million km2, probably result from the mixing of micronutrients into surface waters through the flow of the ACC over the Scotia Arc. This production is consumed by a range of species including Antarctic krill, which are the major prey item of large seabird and marine mammal populations. The flow of the ACC is steered north by the Scotia Arc, pushing polar water to lower latitudes, carrying with it krill during spring and summer, which subsidize food webs around South Georgia and the northern Scotia Arc. There is also marked interannual variability in winter sea ice distribution and sea surface temperatures that is linked to southern hemisphere-scale climate processes such as the El Niño–Southern Oscillation. This variation affects regional primary and secondary production and influences biogeochemical cycles. It also affects krill population dynamics and dispersal, which in turn impacts higher trophic level predator foraging, breeding performance and population dynamics. The ecosystem has also been highly perturbed as a result of harvesting over the last two centuries and significant ecological changes have also occurred in response to rapid regional warming during the second half of the twentieth century. This combination of historical perturbation and rapid regional change highlights that the Scotia Sea ecosystem is likely to show significant change over the next two to three decades, which may result in major ecological shifts.

Warming-induced reductions in body size are greater in aquatic than terrestrial species

Most ectothermic organisms mature at smaller body sizes when reared in warmer conditions. This phenotypically plastic response, known as the “temperature-size rule” (TSR), is one of the most taxonomically widespread patterns in biology. However, the TSR remains a longstanding life-history puzzle for which no dominant driver has been found. We propose that oxygen supply plays a central role in explaining the magnitude of ectothermic temperature-size responses. Given the much lower oxygen availability and greater effort required to increase uptake in water vs. air, we predict that the TSR in aquatic organisms, especially larger species with lower surface area–body mass ratios, will be stronger than in terrestrial organisms. We performed a meta-analysis of 1,890 body mass responses to temperature in controlled experiments on 169 terrestrial, freshwater, and marine species. This reveals that the strength of the temperature-size response is greater in aquatic than terrestrial species. In animal species of ∼100 mg dry mass, the temperature-size response of aquatic organisms is 10 times greater than in terrestrial organisms (−5.0% °C−1 vs. −0.5% °C−1). Moreover, although the size response of small (<0.1 mg dry mass) aquatic and terrestrial species is similar, increases in species size cause the response to become increasingly negative in aquatic species, as predicted, but on average less negative in terrestrial species. These results support oxygen as a major driver of temperature-size responses in aquatic organisms. Further, the environment-dependent differences parallel latitudinal body size clines, and will influence predicted impacts of climate warming on food production, community structure, and food-web dynamics.

Growth and Development Rates Have Different Thermal Responses

Growth and development rates are fundamental to all living organisms. In a warming world, it is important to determine how these rates will respond to increasing temperatures. It is often assumed that the thermal responses of physiological rates are coupled to metabolic rate and thus have the same temperature dependence. However, the existence of the temperature-size rule suggests that intraspecific growth and development are decoupled. Decoupling of these rates would have important consequences for individual species and ecosystems, yet this has not been tested systematically across a range of species. We conducted an analysis on growth and development rate data compiled from the literature for a well-studied group, marine pelagic copepods, and use an information-theoretic approach to test which equations best describe these rates. Growth and development rates were best characterized by models with significantly different parameters: development has stronger temperature dependence than does growth across all life stages. As such, it is incorrect to assume that these rates have the same temperature dependence. We used the best-fit models for these rates to predict changes in organism mass in response to temperature. These predictions follow a concave relationship, which complicates attempts to model the impacts of increasing global temperatures on species body size.

Pelagic production at the Celtic Sea shelf break

This paper reviews the data obtained in the OMEX I Project on biological production in the surface waters of the Celtic Sea shelf break. The study focused on two regions— the Goban Spur and La Chapelle Bank. Satellite images of the Celtic Sea frequently show a region of cooler water at the shelf break, which results in the mixing of cooler, nutrient-rich waters to the sea surface. To examine the hypothesis that the Celtic Sea shelf break might be a region of enhanced production and sedimentation, observations were made at five regions. These were four sites along a transect of the Goban Spur, from the Celtic Sea shelf (water depth <200xa0m), through stations at water depths of 500–1000, 1500, and 3600xa0m; the fifth region was at La Chapelle Bank, which offered a contrasting site where the slope is steeper and influenced by canyons. n nEstimates are made of seasonal production of phytoplankton, bacterioplankton, microzooplankton, and mesozooplankton. The region has a spring bloom which is of short duration at the oceanic sites and occurs earliest on the Celtic Sea shelf; phytoplankton biomass in the summer months is greatest at La Chapelle Bank. Photosynthetic pigments analyses indicate that prymnesiophytes are present throughout the year and are often the dominant group of phytoplankton; diatoms are most abundant in the spring bloom. Primary production is estimated to be ca. 160xa0gCxa0m−2xa0a−1, with cells <5xa0μm in diameter accounting for almost half of the annual primary production. New production is estimated to be equivalent to 80xa0gxa0Cxa0m−2xa0a−1; the f-ratio is generally <0.25 during the summer and autumn months, 0.7–0.8 during the spring bloom, and ca. 0.5 during the winter. n nMicrozooplankton biomass and herbivory were measured from April to October at the Goban Spur regions. The biomass of mesozooplankton was determined from the records of the Continuous Plankton Recorder (CPR) survey, and was used to estimate the amount of primary production removed by mesozooplankton grazing. Bacterial production is estimated to be ca. 12xa0gxa0Cxa0m−2xa0a−1. The sum of microzooplankton and mesozooplankton grazing and the carbon demands of bacteria were significantly lower than primary production from November through May, but heterotrophic processes were quantitatively greater than phytoplankton production from July to October. The data suggest that up to 62xa0gxa0Cxa0m−2xa0a−1 of primary production was not grazed by micro- or mesozooplankton in the surface mixed layer, or utilised directly by bacteria. Depending on the region, up to 38% of the primary production at the Celtic Sea margin was apparently not grazed in the surface mixed layer and would be available for heterotrophic organisms in mid-water and the benthos. The estimated respiration of the heterotrophic community of the surface mixed layer estimated also suggested that between 37% and 60% of the carbon fixed by photosynthesis in the euphotic zone was not remineralised in the surface mixed layer. n nData from satellite remote sensing are used in conjunction with the experimental data to extend the seasonal coverage of the observations made in OMEX I. The archive of the coastal zone color scanner provides mean monthly values of chlorophyll concentration, and these agree well with the seasonal variation of “green colour” of the CPR survey. Primary production has been estimated from the satellite-derived chlorophyll concentrations for the period April–September and is calculated to be 90xa0gxa0Cxa0m−2 for the 6-month period; the estimated production for the same period from in situ experiments suggests that primary production was ca. 116xa0gxa0Cxa0m−2. Nitrate concentrations in the surface water were correlated with sea-surface temperature, and this relationship was applied to temperature measurements from the advanced very high resolution radiometer sensor to estimate the potential nitrate concentrations over the region. The f-ratio was related to nitrate concentration by a simple hyperbolic function (r2=0.73). which was applied to the images of potential nitrate concentration for the region to estimate new production based on satellite data. For the period April through September, new production was calculated to be 46xa0gxa0Cxa0m−2 from satellite estimates of temperature, nitrate, and f-ratio, which compares favourably with the estimated new production of 57xa0gxa0Cxa0m−2 by direct measurement.

A Synthesis of Growth Rates in Marine Epipelagic Invertebrate Zooplankton

We present the most extensive study to date of globally compiled and analysed weight-specific growth rates in marine epi-pelagic invertebrate metazoan zooplankton. Using specified selection criteria, we analyse growth rates from a variety of zooplanktonic taxa, including both holo- and mero-planktonic forms, from over 110 published studies. Nine principal taxonomic groups are considered, the copepods (number of individual data points (n) = 2,528); crustaceans other than copepods (n = 253); cnidarians (n = 77); ctenophores (n = 27); chaetognaths (n = 87); pteropods (n = 8); polychaetes (n = 12); thaliaceans (n = 88); and larvaceans (n = 91). The copepods are further examined by subdividing them into broadcasters or sac-spawning species, and as nauplii (N1-N6), copepodites (C1-C5) and adults (C6). For each taxonomic group relationships between growth, temperature and body weight are examined using a variety of methods. Weight-specific growth tends to increase with increasing temperature and with decreasing body weight in the crustacean group. Growth does not relate to body weight in the case of chaetognaths and larvaceans, but does increase with temperature. In the cnidarian and ctenophore groups growth does not relate to temperature, but is negatively related to body size. For the thaliceans growth increases with both increasing body weight and temperature. In the entire broadcasting copepod data set, weight-specific growth increases with increasing temperature and decreasing body weight. In sac-spawners, growth increases with increasing temperature, and increases with decreasing body weight at temperatures below 20 degrees C, but decreases with body weight at temperatures above this. Comparison between the different taxa shows important differences and similarities. Our extensive synthesis of data generally confirms that larvaceans, pteropods, cnidarians and ctenophores have rates of weight-specific growth that are typically greater than the copepods, chaetognaths and other crustaceans of similar carbon weight. For the cnidarians, ctenophores and larvaceans growth rates are almost always greater than the general relationship describing copepod growth, and are also at the upper limits or beyond the maximum rates for copepods of a similar weight. For the pteropods, growth rates are generally greater than those of copepods, although the data set was limited to a single carnivorous species in a single study (i.e. Clione limacina). The thaliaceans have the highest growth rates for animals with body weights greater than around 1 mg C ind-1, with rates of up to 2.1 d-1 for Pegea bicaudata. Whilst the larvaceans can achieve rates of 2 d-1 in warm tropical waters (28 degrees C), and as high as > 3 d-1 for < 0.2 mg C individual-1 animals of Oikopleura diocia. These are possibly the highest rates every recorded in epi-pelagic metazoans. Reasons for the differences between taxonomic groups are discussed in relation to intrinsic and extrinsic factors and limitations. The importance of this investigation not only lies in it being the most comprehensive overview of patterns of growth to date, but because the data set highlight the gaps in measurements and current knowledge. We examine the inadequacies in the current data sets, and in the methods being used to measure growth and production. Most of the data are for animals collected from coastal and estuarine waters, and it is clear that for a fuller understanding there is an urgent need for work in the open ocean, and for investigations outside temperate regions. There is also a need to explore the role of food availability, and how food concentrations in incubations, and under food saturation, relate to those experienced in the natural environment.

Temperature-size responses match latitudinal-size clines in Arthropods, revealing critical differences between aquatic and terrestrial species

Two major intraspecific patterns of adult size variation are plastic temperature-size (T-S) responses and latitude-size (L-S) clines. Yet, the degree to which these co-vary and share explanatory mechanisms has not been systematically evaluated. We present the largest quantitative comparison of these gradients to date, and find that their direction and magnitude co-vary among 12 arthropod orders (r(2) = 0.72). Body size in aquatic species generally reduces with both warming and decreasing latitude, whereas terrestrial species have much reduced and even opposite gradients. These patterns support the prediction that oxygen limitation is a major controlling factor in water, but not in air. Furthermore, voltinism explains much of the variation in T-S and L-S patterns in terrestrial but not aquatic species. While body size decreases with warming and with decreasing latitude in multivoltine terrestrial arthropods, size increases on average in univoltine species, consistent with predictions from size vs. season-length trade-offs.

Shifts in Mass Scaling of Respiration, Feeding, and Growth Rates across Life-Form Transitions in Marine Pelagic Organisms

The metabolic rate of organisms may be viewed as a basic property from which other vital rates and many ecological patterns emerge and that follows a universal allometric mass scaling law, or it may be considered a property of the organism that emerges as a result of the adaptation to the environment, with consequently fewer universal mass scaling properties. Here, we examine the mass scaling of respiration and maximum feeding (clearance and ingestion rates) and growth rates of heterotrophic pelagic organisms over an ∼1015 range in body mass. We show that clearance and respiration rates have life-form-dependent allometries that have similar scaling but different intercepts, such that the mass-specific rates converge on a rather narrow size-independent range. In contrast, ingestion and growth rates follow a near-universal taxa-independent ∼3/4 mass scaling power law. We argue that the declining mass-specific clearance rates with size within taxa is related to the inherent decrease in feeding efficiency of any particular feeding mode. The transitions between feeding mode and simultaneous transitions in clearance and respiration rates may then represent adaptations to the food environment and be the result of the optimization of trade-offs that allow sufficient feeding and growth rates to balance mortality.

Body shape shifting during growth permits tests that distinguish between competing geometric theories of metabolic scaling

Metabolism fuels all of lifes activities, from biochemical reactions to ecological interactions. According to two intensely debated theories, body size affects metabolism via geometrical influences on the transport of resources and wastes. However, these theories differ crucially in whether the size dependence of metabolism is derived from material transport across external surfaces, or through internal resource-transport networks. We show that when body shape changes during growth, these models make opposing predictions. These models are tested using pelagic invertebrates, because these animals exhibit highly variable intraspecific scaling relationships for metabolic rate and body shape. Metabolic scaling slopes of diverse integument-breathing species were significantly positively correlated with degree of body flattening or elongation during ontogeny, as expected from surface area theory, but contradicting the negative correlations predicted by resource-transport network models. This finding explains strong deviations from predictions of widely adopted theory, and underpins a new explanation for mass-invariant metabolic scaling during ontogeny in animals and plants.

Chapter 2 - When Microscopic Organisms Inform General Ecological Theory

Summary Free-living protists and very small metazoans, such as meiofauna or mesozooplankton, are extremely abundant in aquatic habitats and they play a key role in numerous ecosystem processes, including nutrient cycling and supplying energy to higher trophic levels. Assessing their ecological roles is needed to understand natural systems in their own right, but their study can also be used to inform and test general ecological theories. Organisms smaller than 2xa0mm in size are ideal candidates for laboratory or field studies with a theoretical focus, and we illustrate this point with published examples of pioneering and recent work in which the use of protozoans, aquatic fungi or small metazoans has informed theories within population and community ecology and also more synthetic ecological theories that span different levels of biological organisation (e.g. biodiversity–ecosystem functioning relationships and the metabolic theory of ecology). These literature examples highlight how ecological theory has been stimulated by this research and vice versa. Studies of small species might be sufficiently universal that they can also be applied to the ecology of larger organisms and by extension they could offer insights into how ecosystems operate. In the main part of this chapter, we discuss whether these small worlds can represent the larger world by giving three case studies related to three ecological concepts and topics: the temperature-size rule (TSR), allometric scaling of population abundance and biodiversity–ecosystem functioning relationships. In the first two case studies, we discuss if and how protists differ from small metazoans; in all three case studies we explore if patterns observed for small organisms vary from those detected for larger organisms and the consequences this might have for detecting unifying ecological principles. We used a combination of our own and published data to test predictions related to the TSR and allometric scaling. In the first case, we found that the mechanisms which generate the phenomenon of larger adult size at low temperatures are likely to be fundamentally different for protozoans and multicellular animals. In our second case study, we regressed body size of protists and small metazoans from a large data set against their population abundance. We found that single-celled organisms also differed from small metazoans with regard to their allometries and that both groups differed from larger metazoans. In both these cases, there is evidence that extrapolations from the small to the larger world need to be treated with some caution. In the final case study, we explore to what extent biodiversity–ecosystem functioning relationships among small organisms (aquatic fungi) are similar to those observed for larger organisms.

Impacts of geophysical seismic surveying on fishing success

Over many years acoustic sources of various types have been used in the search for oil and gas in the marine environment. These sources have included sub-marine explosions and airgun blasts, which in turn have been shown in the laboratory, in large scale enclosures, and in situ, to have lethal and sub-lethal effects upon marine mammals, birds and fishes (Richmond and Jones, 1973; Yelverton et al., 1973; Sakaguchi et al., 1976; Wright, 1982; Linton et al., 1985; Sverdrup et al., 1994; Goold, 1996). Although explosive charges were commonly used until the 1960s, by 1985, 97% of seismic surveys used airgun devices (Holliday et al., 1987), and we therefore concentrate on the latter in this review. Organisms may not only be immediately killed on exposure to airgun detonations (Turnpenny and Nedwell, 1994; McCauley, 1994), but their mortality may also be delayed as a result of direct physiological damage, or indirectly from increased predation. The effects of close range airgun discharge on short-term (i.e. minutes to days) mortality of eggs, juvenile and adult fish have been examined in some detail and reviewed in Turnpenny and Nedwell (1994), and impacts of airguns at close range are briefly considered herein for completeness. Our objective is to review effects on a larger spatial and temporal scale than is typical for close range studies, specifically for the first time bring together published information on fish catch success.