Inferred support for disturbance-recovery hypothesis of North Atlantic phytoplankton blooms

Analyses of satellite-derived chlorophyll data indicate that the phase of rapid phytoplankton population growth in the North Atlantic (the ‘spring bloom') is actually initiated in the winter rather than the spring, contradicting Sverdrup's Critical Depth Hypothesis. An alternative disturbance-recovery hypothesis (DRH) has been proposed to explain this discrepancy, in which the rapid deepening of the mixed layer reduces zooplankton grazing rates sufficiently to initiate the bloom. We use Bayesian parameter inference on a simple Nutrient-Phytoplankton-Zooplankton (NPZ) to investigate the DRH and also investigate how well the model can capture the multiyear and spatial dynamics of phytoplankton concentrations and population growth rates. Every parameter in our NPZ model was inferred as a probability distribution given empirical constraints, this provides a more objective method to identify a model parameterisation given available empirical evidence, rather than fixing or tuning individual parameter values. Our model explains around 75% of variation in the seasonal dynamics of phytoplankton concentrations, 30% of variation in their population rates of change, and correctly predicts the phases of population growth and decline. Our parameter-inferred model supports DRH, revealing the sustained reduction of grazing due to mixed layer deepening as the driving mechanism behind bloom initiation, with the relaxation of nutrient limitation being another contributory mechanism. Our results also show that the continuation of the bloom is caused in part by the maintenance of phytoplankton concentrations below a level that can support positive zooplankton population growth. Our approach could be employed to formally assess alternative hypotheses for bloom formatio

Spatial and temporal variability and connectivity of the marine environment of the South Sandwich Islands, Southern Ocean

The South Sandwich Islands form the eastern boundary to the highly biologically productive Scotia Sea in the southwest Atlantic sector of the Southern Ocean and are part of a large Marine Protected Area. The South Sandwich Islands have a complex marine environment that is influenced by both the Antarctic Circumpolar Current and the Weddell Gyre, and seasonal sea ice. Here we investigate the local and regional dynamics and variability of the ocean and sea ice to inform management of the region. Remotely sensed sea surface temperature (SST), sea ice concentration and chlorophyll a data from 2009 to 2021 are used to define the mean seasonal cycle in the environment and the associated temporal and spatial variability. While sea surface temperature and sea ice have a clearly defined seasonality, local chlorophyll blooms are irregular in both timing, location and magnitude. Interannual variability in SST is strongly positively correlated along the island arc. The islands experience very different winter sea ice conditions from year to year, with marked variability in sea ice distribution and duration. Surface chlorophyll blooms develop in most years close to the island arc, but there is little spatial consistency and there are years where blooms are not observed. The timing and pattern of sea ice retreat appears to be a key driver in the formation of chlorophyll blooms, with their propagation affected by local circulation, but additional local processes are also important. Trajectories of near-surface satellite tracked surface buoys and Argo floats, together with an analysis of sea surface height output from a global reanalysis product, demonstrate the connectivity of the South Sandwich Islands to the wider regional marine system. Enhanced current flows around and between the South Sandwich Islands are likely to affect the transport and exchange of material along the island arc. The South Sandwich Islands are connected with the Scotia and Weddell seas, with contribution from the different regions varying according to latitude along the island arc. There are also connections with islands downstream including Bouvet, Crozet and Kerguelen Islands and seamounts, with possible return flow via the Weddell Gyre. Our analyses indicate that accounting for the complexity and variability in the South Sandwich Islands marine environment will be crucial in the development of conservation and fisheries management procedures

Status, Change, and Futures of Zooplankton in the Southern Ocean

In the Southern Ocean, several zooplankton taxonomic groups, euphausiids, copepods, salps and pteropods, are notable because of their biomass and abundance and their roles in maintaining food webs and ecosystem structure and function, including the provision of globally important ecosystem services. These groups are consumers of microbes, primary and secondary producers, and are prey for fishes, cephalopods, seabirds, and marine mammals. In providing the link between microbes, primary production, and higher trophic levels these taxa influence energy flows, biological production and biomass, biogeochemical cycles, carbon flux and food web interactions thereby modulating the structure and functioning of ecosystems. Additionally, Antarctic krill (Euphausia superba) and various fish species are harvested by international fisheries. Global and local drivers of change are expected to affect the dynamics of key zooplankton species, which may have potentially profound and wide-ranging implications for Southern Ocean ecosystems and the services they provide. Here we assess the current understanding of the dominant metazoan zooplankton within the Southern Ocean, including Antarctic krill and other key euphausiid, copepod, salp and pteropod species. We provide a systematic overview of observed and potential future responses of these taxa to a changing Southern Ocean and the functional relationships by which drivers may impact them. To support future ecosystem assessments and conservation and management strategies, we also identify priorities for Southern Ocean zooplankton research

Restricted regions of enhanced growth of Antarctic krill in the circumpolar Southern Ocean

Food webs in high-latitude oceans are dominated by relatively few species. Future ocean and sea-ice changes affecting the distribution of such species will impact the structure and functioning of whole ecosystems. Antarctic krill (Euphausia superba) is a key species in Southern Ocean food webs, but there is little understanding of the factors influencing its success throughout much of the ocean. The capacity of a habitat to maintain growth will be crucial and here we use an empirical relationship of growth rate to assess seasonal spatial variability. Over much of the ocean, potential for growth is limited, with three restricted oceanic regions where seasonal conditions permit high growth rates, and only a few areas around the Scotia Sea and Antarctic Peninsula suitable for growth of the largest krill (>60 mm). Our study demonstrates that projections of impacts of future change need to account for spatial and seasonal variability of key ecological processes within ocean ecosystems

Bulk element compositions of meteorites: A guide for interpreting remote-sensing geochemical measurements of planets and asteroids

We report a large database of bulk meteorite elemental abundances, compiled to aid in the interpretation of elemental abundance data determined by remote-sensing instrumentation on planetary missions. A custom user interface was developed for easy access and manipulation of the abundance data. The database contains almost 3000 individual analyses of more than 1000 individual meteorites. Most major and minor elements are included, as well as small number of trace elements measurable by remote-sensing gamma-ray spectroscopy (notably Th and U). All meteorite classes show variability in bulk compositions between individual analyses. Some of this spread is intrinsic to the parent bodies of the meteorites. However, some variability is undoubtedly due to systematic uncertainties, caused by inter-laboratory bias, misclassification, effect of weathering, and unrepresentative sampling. We use the database here to investigate both how well different meteorite groups can be distinguished on the basis of bulk compositions and how bulk compositions can be related to the cosmochemical and geological processes that produced them. The major elements measurable by X-ray and gamma-ray remote-sensing-oxygen, magnesium, aluminum, silicon, sulfur, calcium and iron-reflect to differing degrees nebular elemental fractionations and parent-body igneous processes and can be used together to distinguish most classes and sub-classes of meteorites. Potassium is potentially useful as a tracer of thermal processes in the early solar system. Thorium and uranium abundances could be used to trace igneous processes on differentiated asteroids

Circumpolar patterns in Antarctic krill larval recruitment: an environmentally-driven model

Larval recruitment in Antarctic krill is known to be episodic and regional. We consider the importance of a range of recruitment factors using an environmentally driven model of larval development from spawning to post-larvae. Our model examines the timing of spawning, interaction with bathymetry, susceptibility to cold temperatures, temperature-driven development and the seasonal cycle of sea ice to identify those factors with the greatest impact. The model predicts that the seasonal location of sea ice is the main limiting factor for successful larval recruitment. Spawning in January leads to the greatest area of viable larval recruitment habitat. Dense sea ice cover, which we assume that adult krill do not spawn under, prevents spawning in large areas early in the breeding season (December). Nevertheless, later spawning in February, when sea ice is at a minimum, means there is often insufficient time for the larvae to reach a viable developmental stage before the sea ice advances. Meanwhile, although spawning is possible in more northerly areas throughout the breeding season, these are generally remote from winter sea ice, which is assumed to be necessary for larvae to overwinter. Interaction with bathymetry before hatching further limits suitable habitat. Over a 12 yr period, the model predicted larval re - cruitment from January spawning in all years in the Cooperation, Ross and Weddell Seas, with episodic larval recruitment in the Bransfield Strait in 9 of the 12 years. Additional understanding of the overwintering requirements of larvae, together with regional studies at higher spatial resolution, particularly in shelf regions, will better constrain the uncertainties in the model

Myctophid fish (Family Myctophidae) are central consumers in the food web of the Scotia Sea (Southern Ocean)

Myctophid fish are the most abundant and diverse mesopelagic fishes in the Southern Ocean. They are a conduit of energy between primary consumers and higher marine predators, and between the upper surface layers and the mesopelagic depths. However, there remain major uncertainties about their ecology, particularly regarding their role in Southern Ocean food webs, which are often regarded as dominated by Antarctic krill in waters south of the Antarctic Polar Front. Limited data on the feeding ecology of myctophids has made it difficult to assess the importance of myctophids as consumers of krill and how they fit in the traditional view of a krill-dominated system (diatom-krill-higher predator). We provide a new assessment of the role of myctophids in Southern Ocean food webs using information from recent trophodynamic studies of myctophids conducted in the Scotia Sea, one of the most productive regions of the Southern Ocean and a region that sustains both major populations of higher predators (sea birds, seals, whales) and important commercial fisheries (krill, toothfish and mackerel icefish). Collectively, these data show that myctophids have a central role in Southern Ocean food webs as both predators and prey. Large myctophid species are prevalent consumers of krill throughout their distributional range and in different seasons in the Scotia Sea. Moreover, best estimates of both myctophid and higher predator consumption of krill to date indicate that large myctophids are the greatest predators of krill in this region, consuming almost as much krill as all other vertebrate predators of krill. Nevertheless, there are several smaller myctophid species that do not eat krill, instead consuming copepods and other small euphausiids. Myctophids therefore link primary producers to higher predators through both krill-dependent and krill-independent trophic pathways, emphasizing their importance in regional food webs. Consequently, myctophid-based trophic pathways are unlikely to be exempt from the direct consequences of a redistribution and/or reduction in krill population biomass. The extent to which myctophids can maintain food web stability and sustain higher predator populations during periods of prolonged reductions in krill abundance is considered further

An intercomparison of models predicting growth of Antarctic krill (Euphausia superba): The importance of recognizing model specificity

Antarctic krill (Euphausia superba) is a key species of the Southern Ocean, impacted by climate change and human exploitation. Understanding how these changes affect the distribution and abundance of krill is crucial for generating projections of change for Southern Ocean ecosystems. Krill growth is an important indicator of habitat suitability and a series of models have been developed and used to examine krill growth potential at different spatial and temporal scales. The available models have been developed using a range of empirical and mechanistic approaches, providing alternative perspectives and comparative analyses of the key processes influencing krill growth. Here we undertake an intercomparison of a suite of the available models to understand their sensitivities to major driving variables. This illustrates that the results are strongly determined by the model structure and technical characteristics, and the data on which they were developed and validated. Our results emphasize the importance of assessing the constraints and requirements of individual krill growth models to ensure their appropriate application. The study also demonstrates the value of the development of alternative modelling approaches to identify key processes affecting the dynamics of krill. Of critical importance for modelling the growth of krill is appropriately assessing and accounting for differences in estimates of food availability resulting from alternative methods of observation. We suggest that an intercomparison approach is particularly valuable in the development and application of models for the assessment of krill growth potential at circumpolar scales and for future projections. As another result of the intercomparison, the implementations of the models used in this study are now publicly available for future use and analyses