Acquisition of continuous plankton recorder data

The Continuous Plankton Recorder has been deployed on a seasonal basis in the north Pacific since 2000, accumulating a database of abundance measurements for over 290 planktonic taxa in over 3,500 processed samples. There is an additional archive of over 10,000 samples available for further analyses. Exxon Valdez Oil Spill Trustee Council financial support has contributed to about half of this tally, through four projects funded since 2002. Time series of zooplankton variables for sub-regions of the survey area are presented together with abstracts of eight papers published using data from these projects. The time series covers a period when the dominant climate signal in the north Pacific, the Pacific Decadal Oscillation (PDO), switched with unusual frequency between warm/positive states (pre-1999 and 2003-2006) and cool/negative states (1999-2002 and 2007). The CPR data suggest that cool negative years show higher biomass on the shelf and lower biomass in the open ocean, while the reverse is true in warm (PDO positive) years with lower shelf biomass (except 2005) and higher oceanic biomass. In addition, there was a delay in plankton increase on the Alaskan shelf in the colder spring of 2007, compared to the warmer springs of the preceding years. In warm years, smaller species of copepods which lack lipid reserves are also more common. Availability of the zooplankton prey to higher trophic levels (including those that society values highly) is therefore dependent on the timing of increase and peak abundance, ease of capture and nutritional value. Previously published studies using these data highlight the wide-ranging applicability of CPR data and include collaborative studies on; phenology in the key copepod species Neocalanus plumchrus, descriptions of distributions of decapod larvae and euphausiid species, the effects of hydrographic features such as mesoscale eddies and the North Pacific Current on plankton populations and a molecularbased investigation of macro-scale population structure in N. cristatus. The future funding situation is uncertain but the value of the data and studies so far accumulated is considerable and sets a strong foundation for further studies on plankton dynamics and interactions with higher trophic levels in the northern Gulf of Alaska.

Causes and prediction of abrupt climate-driven ecosystem shifts in the North Atlantic

Warming of the global climate is now unequivocal and its impact on Earth’ functional units has become more apparent. Here, we show that marine ecosystems are not equally sensitive to climate change and reveal a critical thermal boundary where a small increase in temperature triggers abrupt ecosystem shifts seen across multiple trophic levels. This large-scale boundary is located in regions where abrupt ecosystem shifts have been reported in the North Atlantic sector and thereby allows us to link these shifts by a global common phenomenon. We show that these changes alter the biodiversity and carrying capacity of ecosystems and may, combined with fishing, precipitate the reduction of some stocks of Atlantic cod already severely impacted by exploitation. These findings offer a way to anticipate major ecosystem changes and to propose adaptive strategies for marine exploited resources such as cod in order to minimize social and economic consequences.

Spatial changes in the sensitivity of Atlantic cod to climate-driven effects in the plankton

Recent strategies to sustain fish stocks have suggested a move towards an ecosystem based fisheries management (EBFM) approach. While EBFM considers the effect of fishing at the ecosystem level, it generally struggles with climate-driven environmental variability. In this study we show that the position of a fish stock within its distributional range or thermal niche (we use Icelandic and North Sea cod as examples of stocks at the centre and edge of their niche, respectively) will influence the relative importance of fishing and climate on abundance. At the warmer edge of the thermal niche of cod in the North Sea, we show a prominent influence of climate on the cod stock that is mediated through temperature effects on the plankton. In contrast, the influence of climate through its effects on plankton appears much less important at the present centre of the niche around Iceland. Recognising the potentially strong effect of climate on fish stocks, at a time of rapid global climate change, is probably an important prerequisite towards the synthesis of a cod management strategy.

Modelling a light-driven phytoplankton succession

We used a numerical model to investigate if and to what extent cellular photoprotective capacity accounts for succession and vertical distribution of marine phytoplankton species/groups. A model describing xanthophyll photoprotective activity in phytoplankton has been implemented in the European Regional Sea Ecosystem Model and applied at the station L4 in the Western English Channel. Primary producers were subdivided into three phytoplankton functional types defined in terms of their capacity to acclimate to different light-specific environments: low light (LL-type), high light (HL-type) and variable light (VL-type) adapted species. The LL-type is assumed to have low cellular level of xanthophyll-cycling pigments (PX) relative to the modelled photosynthetically active pigments (chlorophyll and fucoxanthin (FUCO) = PSP). The HL-type has high PX content relative to PSP while VL-type presents an intermediate PX to PSP ratio. Furthermore, the VL-type is capable of reversibly converting FUCO to PX and synthesizing new PX under high-light stress. In order to reproduce phytoplankton community succession with each of the three groups being dominant in different periods of the year, we had also to assume reduced grazing pressure on HL-adapted species. Model simulations realistically reproduce the observed seasonal patterns of pigments and nutrients highlighting the reasonability of the underpinning assumptions. Our model suggests that pigment-mediated photophysiology plays a primary role in determining the evolution of marine phytoplankton communities in the winter-spring period corresponding to the shoaling of the mixed layer and the increase of light intensity. Grazing selectivity however contributes to the phytoplankton community composition in summer.

Evolution of alternative biosynthetic pathways for vitamin C following plastid acquisition in photosynthetic eukaryotes.

Ascorbic acid (vitamin C) is an enzyme co-factor in eukaryotes that also plays a critical role in protecting photosynthetic eukaryotes against damaging reactive oxygen species derived from the chloroplast. Many animal lineages, including primates, have become ascorbate auxotrophs due to the loss of the terminal enzyme in their biosynthetic pathway, L-gulonolactone oxidase (GULO). The alternative pathways found in land plants and Euglena use a different terminal enzyme, L-galactonolactone dehydrogenase (GLDH). The evolutionary processes leading to these differing pathways and their contribution to the cellular roles of ascorbate remain unclear. Here we present molecular and biochemical evidence demonstrating that GULO was functionally replaced with GLDH in photosynthetic eukaryote lineages following plastid acquisition. GULO has therefore been lost repeatedly throughout eukaryote evolution. The formation of the alternative biosynthetic pathways in photosynthetic eukaryotes uncoupled ascorbate synthesis from hydrogen peroxide production and likely contributed to the rise of ascorbate as a major photoprotective antioxidant.

Southern Annular Mode and westerly-wind-driven changes in Indian-Atlantic exchange mechanisms

The dynamical link between the Indian Ocean and Atlantic Meridional Overturning Circulation (AMOC) remains poorly understood. This partly arises from the complex Agulhas leakage, which occurs via rings, cyclones, and non-eddy flux. Hindcast simulations suggest that leakage has recently increased but have not decomposed this signal into its constituent mechanisms. Here these are isolated in a realistic ocean model. Increases in simulated leakage are attributed to stronger eddy and non-eddy-driven transports, and a strong warming and salinification, especially within Agulhas rings. Variability in both regimes is associated with strengthening Indian Ocean westerly winds, reflecting an increasingly positive Southern Annular Mode. While eddy and non-eddy flux signals are tied through turbulent eddy dissipation, the ratio between the two varies decadally. Consequently, while altimetry suggests a recent increase in retroflection turbulence and implied leakage, non-eddy flux may also play a significant role in modulating the leakage AMOC connection.

Dynamic responses of the benthic bacterial community at the Western English Channel observatory site L4 are driven by deposition of fresh phytodetritus

The impact of the seasonal deposition of phytoplankton and phytodetritus on surface sediment bacterial abundance and community composition was investigated at the Western English Channel site L4. Sediment and water samples were collected from January to September in 2012, increasing in frequency during periods of high water column phytoplankton abundance. Compared to the past two decades, the spring bloom in 2012 was both unusually long in duration and contained higher than average biomass. Within spring months, the phytoplankton bloom was well mixed through the water column and showed accumulations near the sea bed, as evidenced by flow cytometry measurements of nanoeukaryotes, water column chlorophyll a and the appearance of pelagic phytoplankton at the sediment. Measurements of chlorophyll and chlorophyll degradation products indicated phytoplankton material was heavily degraded after it reached the sediment surface: the nature of the chlorophyll degradation products (predominantly pheophorbide, pyropheophorbide and hydroxychlorophyllone) was indicative of grazing activity. The abundance of bacterial 16S rRNA genes g−1 sediment (used as a proxy for bacterial biomass) increased markedly with the onset of the phytoplankton bloom, and correlated with measurements of chlorophyll at the surface sediment. Together, this suggests that bacteria may have responded to nutrients released via grazing activity. In depth sequencing of the 16S rRNA genes indicated that the composition of the bacterial community shifted rapidly through-out the prolonged spring bloom period. This was primarily due to an increase in the relative sequence abundance of Flavobacteria.