Traits controlling body size in copepods: separating general constraints from species-specific strategies

A new synthesis of laboratory measurements of food-saturated development and growth across diverse copepod taxa was conducted in a theoretical framework that distinguishes general allometric constraints on copepod physiology from contingent strategies that correlate with size for other reasons. After temperature correction, the allometry of growth rate is inconsistent between the ontogeny of Calanus spp., where it follows the classic -0.3 power-law scaling, and a broader spectrum of adult size Wa (0.3 to 2000 µg C, Oithona spp. to Neocalanus spp.), across which the classic scaling appears to represent only an upper limit. Over the full size spectrum, after temperature correction, a growth rate g0relative to the -0.3 power law correlates with adult size better than does relative (temperature-corrected) development rate u0; in contrast, at a finer scale of diversity (among Calanus spp., or among large (\u3e50 µg C) calanoids in general), u0 is the better correlate with adult size and the effect of g0 is insignificant. Across all these scales, the ratio of relative growth and development rates g0/u0 is a better predictor of adult size than g0 or u0 alone, consistent with a simple model of individual growth

Copepod life strategy and population viability in response to prey timing and temperature : testing a new model across latitude, time, and the size spectrum

A new model ("Coltrane": Copepod Life-history Traits and Adaptation to Novel Environments) describes environmental controls on copepod populations via (1) phenology and life history and (2) temperature and energy budgets in a unified framework. The model tracks a cohort of copepods spawned on a given date using a set of coupled equations for structural and reserve biomass, developmental stage, and survivorship, similar to many other individual-based models. It then analyzes a family of cases varying spawning date over the year to produce population-level results, and families of cases varying one or more traits to produce community-level results. In an idealized global-scale testbed, the model correctly predicts life strategies in large Calanus spp. ranging from multiple generations per year to multiple years per generation. In a Bering Sea testbed, the model replicates the dramatic variability in the abundance of Calanus glacialis/marshallae observed between warm and cold years of the 2000s, and indicates that prey phenology linked to sea ice is a more important driver than temperature per se. In a Disko Bay, West Greenland testbed, the model predicts the viability of a spectrum of large-copepod strategies from income breeders with a adult size ~100 μgC reproducing once per year through capital breeders with an adult size > 1000 μgC with a multiple-year life cycle. This spectrum corresponds closely to the observed life histories and physiology of local populations of Calanus finmarchicus, C. glacialis, and Calanus hyperboreus. Together, these complementary initial experiments demonstrate that many patterns in copepod community composition and productivity can be predicted from only a few key constraints on the individual energy budget: the total energy available in a given environment per year; the energy and time required to build an adult body; the metabolic and predation penalties for taking too long to reproduce; and the size and temperature dependence of the vital rates involved

Projected impacts of 21st century climate change on diapause in Calanus finmarchicus

Diapause plays a key role in the life cycle of high latitude zooplankton. During diapause animals avoid starving in winter by living in deep waters where metabolism is lower and met by lipid reserves. Global warming is therefore expected to shorten the maximum potential diapause duration by increasing metabolic rates and by reducing body size and lipid reserves. This will alter the phenology of zooplankton, impact higher trophic levels and disrupt biological carbon pumps. Here we project the impacts of climate change on the key North Atlantic copepod Calanus finmarchicus under IPCC RCP 8.5. Potential diapause duration is modelled in relation to body size and overwintering temperature. The projections show pronounced geographic variations. Potential diapause duration reduces by more than 30% in the Western Atlantic, whereas in the key overwintering centre of the Norwegian Sea it changes only marginally. Surface temperature rises, which reduce body size and lipid reserves, will have a similar impact to deep water changes on diapause in many regions. Because deep water warming lags that at the surface, animals in the Labrador Sea could offset warming impacts by diapausing in deeper waters. However, the ability to control diapause depth may be limited

Eat or sleep : availability of winter prey explains mid-winter and spring activity in an arctic Calanus population

Copepods of the genus Calanus have adapted to high levels of seasonality in prey availability by entering a period of hibernation during winter known as diapause, but repeated observations of active Calanus spp. have been made in January in high latitude fjords which suggests plasticity in over-wintering strategies. During the last decade, the period of Polar Night has been studied intensively in the Arctic. A continuous presence of an active microbial food web suggests the prevalence of low-level alternative copepod prey (such as microzooplankton) throughout this period of darkness. Here we provide further evidence of mid-winter zooplankton activity using a decadal record of moored acoustics from Kongsfjorden, Svalbard. We apply an individual based life-history model to investigate the fitness consequences of a range of over-wintering strategies (in terms of diapause timing and duration) under a variety of prey availability scenarios. In scenarios of no winter prey availability ((Formula presented.)), the optimal time to exit diapause is in March. However, as P win increases (up to 40μgCL −1), there is little fitness difference in copepods exiting diapause in January compared to March. From this, we suggest that Calanus are able (in energetic terms) to either i) exit diapause early to deal with uncertainty in spring bloom timing, or ii) remain active throughout winter if diapause is not possible (i.e., environment not deep enough, or not enough lipid reserves built up over the previous summer). The range of viable overwintering strategies increases with increasing P win, suggesting that there is more flexibility for Calanus spp. in a scenario of non-zero P win

Multi-day water residence time as a mechanism for physical and biological gradients across intertidal flats

Tidal flats with shallow-sloping bathymetry under meso-to macrotidal conditions allow organisms to occupy similar tidal elevations at different distances from subtidal channels. As water floods or ebbs across such tidal flats during a single tidal cycle, upstream organisms may modify water properties such as chlorophyll concentration, while physiochemical properties may change due to close association with sediments. Here we report evidence for an additional mechanism establishing cross-shore gradients: multi-day water residence times, in the sense that even if water completely drains into subtidal channels at low tide, a large fraction returns to the flats on the next high tide. We applied circulation modeling and empirical measurements of water properties and benthic secondary production to a 1-km-wide tidal flat in Willapa Bay, Washington, USA. From the circulation model, water parcels on this intertidal flat have residence times up to 2 d, that is, water found on the flat at one high tide returns to the intertidal zone for a median of 4 successive semidiurnal high tides. Modeled residence times generally increased towards shore. Four empirical datasets showed cross-shore gradients consistent with modeled residence times: Salinity time series lagged towards shore; water column chlorophyll declined towards shore at fixed stations (near-bottom) and in surface transects more than could be explained by benthic suspension-feeding during a single transit of water; and oyster (Magallana = Crassostrea gigas) condition declined 25% over 0.5 km from channel to shore, independent of tidal elevation. One environmental measurement was more consistent with within-tide change, as water temperatures warmed towards shore on afternoon flood tides but showed no tidal-cycle lags. Taken together, these patterns suggest that multi-day water residence times can contribute to environmental heterogeneity from channel to shore on tidal flats, acting orthogonally to well-recognized estuarine gradients in residence time from ocean to river

Sea ice decline drives biogeographical shifts of key Calanus species in the central Arctic Ocean

In recent decades, the central Arctic Ocean has been experiencing dramatic decline in sea ice coverage, thickness and extent, which is expected to have a tremendous impact on all levels of Arctic marine life. Here, we analyze the regional and temporal changes in pan-Arctic distribution and population structure of the key zooplankton species Calanus glacialis and C. hyperboreus in relation to recent changes in ice conditions, based on historical (1993–1998) and recent (2007–2016) zooplankton collections and satellite-based sea ice observations. We found strong correlations between Calanus abundance/population structure and a number of sea ice parameters. These relationships were particularly strong for C. glacialis, with higher numbers being observed at locations with a lower ice concentration, a shorter distance to the ice edge, and more days of open water. Interestingly, early stages of C. hyperboreus followed the same trends, suggesting that these two species substantially overlap in their core distribution area in the Arctic Ocean. Calanus glacialis and C. hyperboreus have been historically classified as shelf versus basin species, yet we conclude that both species can inhabit a wide range of bottom depths and their distribution in the Arctic Ocean is largely shaped by sea ice dynamics. Our data suggest that the core distribution patterns of these key zooplankton are shifting northwards with retreating sea ice and changing climate conditions.publishedVersio

Sea ice decline drives biogeographical shifts of key Calanus species in the central Arctic Ocean

publishedVersio

The onset of the spring phytoplankton bloom in the coastal North Sea supports the Disturbance Recovery Hypothesis

The spring phytoplankton bloom is a key event in temperate and polar seas, yet the mechanisms that trigger it remain under debate. Some hypotheses claim that the spring bloom onset occurs when light is no longer limiting, allowing phytoplankton division rates to surpass a critical threshold. In contrast, the Disturbance Recovery Hypothesis (DRH) proposes that the onset responds to an imbalance between phytoplankton growth and loss processes, allowing phytoplankton biomass to start accumulating, and this can occur even when light is still limiting. Although several studies have shown that the DRH can explain the spring bloom onset in oceanic waters, it is less certain whether and how it also applies to coastal areas. To address this question at a coastal location in the Scottish North Sea, we combined 21 years (1997-2017) of weekly in situ chlorophyll and environmental data with meteorological information. Additionally, we also analyzed phytoplankton cell counts estimated using microscopy (2000-2017) and flow cytometry (2015-2017). The onset of phytoplankton biomass accumulation occurred around the same date each year, 16 ± 11 d (mean ± SD) after the winter solstice, when light limitation for growth was strongest. Also, negative and positive biomass accumulation rates (r) occurred respectively before and after the winter solstice at similar light levels. The seasonal change from negative to positive r was mainly driven by the rate of change in light availability rather than light itself. Our results support the validity of the DRH for the studied coastal region and suggest its applicability to other coastal areas

Estimating behavior in a black box : how coastal oceanographic dynamics influence yearling Chinook salmon marine growth and migration behaviors

Ocean currents or temperature may substantially influence migration behavior in many marine species. However, high-resolution data on animal movement in the marine environment are scarce; therefore, analysts and managers must typically rely on unvalidated assumptions regarding movement, behavior, and habitat use. We used a spatially explicit, individual-based model of early marine migration with two stocks of yearling Chinook salmon to quantify the influence of external forces on estimates of swim speed, consumption, and growth. Model results suggest that salmon behaviorally compensate for changes in the strength and direction of ocean currents. These compensations can result in salmon swimming several times farther than their net movement (straight-line distance) would indicate. However, the magnitude of discrepancy between compensated and straight-line distances varied between oceanographic models. Nevertheless, estimates of relative swim speed among fish groups were less sensitive to the choice of model than estimates of absolute individual swim speed. By comparing groups of fish, this tool can be applied to management questions, such as how experiences and behavior may differ between groups of hatchery fish released early vs. later in the season. By taking into account the experiences and behavior of individual fish, as well as the influence of physical ocean processes, our approach helps illuminate the “black box” of juvenile salmon behavior in the early marine phase of the life cycle