Changes in the oceanic northeast Pacific plankton populations; what may happen in a warmer ocean

The Continuous Plankton Recorder has been sampling the northeast Pacific on a routine basis since 2000. Although this is a relatively short time series still, climate variability within that time has caused noticeable related changes in the plankton. The earlier part of the time series followed the 1999 La Nina and conditions were cool, but conditions between 2003 and 2005 were anomalously warm. Oceanic zooplankton have responded to this warming in several ways that are discernible in CPR data. The seasonal cycle of mesozooplankton biomass in the eastern Gulf of Alaska has shifted earlier in the spring by a few weeks (sampling resolution is too coarse to be more accurate). The copepod Neocalanus plumchruslflemingeri is largely responsible as it makes up a high proportion of the spring surface biomass and stage-based determinations have shown an earlier maximum in warmer years across much of the northeast Pacific, spanning nearly 20 degrees of latitude. Summer copepod populations are more diverse than in spring, although lower in biomass. The northwards extension of southern taxa in the summer correlates with surface temperature and in warmer years southern taxa are found further north than in cooler years. These findings support the importance of monitoring the open ocean particularly as it is an important foraging ground for large fish, birds and mammals. Higher trophic levels may time their reproduction or migration to coincide with the abundance of particular prey which may be of a different composition and/or lower abundance at a particular time in warmer conditions.

The North Pacific Continuous Plankton Recorder Survey

The Continuous Plankton Recorder (CPR) survey of the North Pacific is a PICES project now in its ninth year and facing an uncertain future. CPRs have been towed behind commercial ships along two (north–south and east–west) transects for a total of ~ nine times per year. Samples are collected with a filtering mesh and are then microscopically processed for plankton abundance in the laboratory. The survey, so far, has accumulated 3,648 processed samples (with approximately three times as many archived without processing), each representing 18 km of the transect (Fig. 1) and containing an abundance of data on over 290 phytoplankton and zooplankton taxa. A CTD with a fluorometer has been attached to the CPR sampling at the east–west transect in more recent years to provide supplementary environmental data.

Plankton populations at the bifurcation of the North Pacific Current

As the eastward-flowing North Pacific Current approaches the North American continent it bifurcates into the southward-flowing California Current and the northward-flowing Alaska Current. This bifurcation occurs in the south-eastern Gulf of Alaska and can vary in position. Dynamic height data from Project Argo floats have recently enabled the creation of surface circulation maps which show the likely position of the bifurcation; during 2002 it was relatively far north at 53 degrees N then, during early 2003, it moved southwards to a more normal position at 45 degrees N. Two ship-of-opportunity transects collecting plankton samples with a Continuous Plankton Recorder across the Gulf of Alaska were sampled seasonally during 2002 and 2003. Their position was dependent on the commercial ship's operations; however, most transects sampled across the bifurcation. We show that the oceanic plankton differed in community composition according to the current system they occurred in during spring and fall of 2002 and 2003, although winter populations were more mixed. Displacement of the plankton communities could have impacts on the plankton's reproduction and development if they use cues such as day length, and also on foraging of higher trophic-level organisms that use particular regions of the ocean if the nutritional value of the communities is different. Although we identify some indicator taxa for the Alaska and California currents, functional differences in the plankton communities on either side of the bifurcation need to be better established to determine the impacts of bifurcation movement on the ecosystems of the north-east Pacific.

Changes in development timing and cohort width of Neocalanus plumchrus degree ' flemingeri copepods in the eastern North Pacific

Neocalanus plumchrus/flemingeri copepods make up a large proportion of spring mesozooplankton biomass and are a valuable nutritional source for many higher trophic levels. Copepodites through to sub-adult stage are present in surface waters for a relatively short period of time each spring, and the date of maximum biomass has been calculated as the date when 50% of the population were at the sub-adult, CV stage. This index allows quite a precise date to be calculated from relatively infrequent sampling and interannual comparisons between 1957 and 2004 have demonstrated that the timing of peak abundance is significantly advanced in warmer years. However, recent data from the Continuous Plankton Recorder survey, which samples the surface NE Pacific more frequently during spring, has found that maximum numbers of CV copepodites occur after the 50% point is reached so that maximum biomass occurs some weeks later than predicted by this index (although comparisons between years show that the magnitude of the timing shift is similar). Comparisons with depth-stratified profiles from the BIONESS show that this is not just due to single-depth near-surface sampling by the CPR. We speculate on the cause of this change which could be related to the width of the cohort (which appears to now be narrower, at least in warm years) or the length of time that the CV stage needs to spend in the surface accumulating lipid before beginning diapause. A narrower cohort has implications for predators who will have less time to take advantage of this food source.

Shortened duration of the annual Neocalanus plumchrus biomass peak in the Northeast Pacific

The calanoid copepod Neocalan us plumchrus (Marukawa) is a dominant member of the spring mesozooplankton in the subarctic North Pacific and Bering Sea. Previous studies have shown interdecadal and latitudinal variation in seasonal developmental timing, with peak biomass occurring earlier in years and places with warmer upper ocean temperatures. Because N. plumchrus normally has a single dominant annual cohort, its seasonal timing can be indexed from measurements of total population biomass or by following progressive changes in stage composition. Early studies empirically found that peak upper ocean biomass occurred when about half of the pre-dormant population had reached copepodite stage 5 (C5). However, more recent comparisons derived from recent Continuous Plankton Recorder (CPR) data now show peak biomass when a larger fraction (> 80%) of the population is at C5. CPR samples the surface 10 to 15 m, but comparisons to depth-resolved BIONESS data show that this discrepancy is not an artefact of sampling depth. Other causes are either a prolongation of duration of pre-dormant C5 or a narrowing of the age range making up the annual cohort. We assessed changes in cohort width using a modification of Greve's cumulative percentile method, and found that average cohort widths in the Alaska Gyre were significantly narrower in 2000-2007 than in 1957-1965 (1968-1980 were intermediate). Net tow sampling of Strait of Georgia populations showed a similar significant narrowing of cohorts in the 2003-2005 sampling period. This study provides evidence that in addition to the shift to an earlier occurrence of peak biomass reported previously, the duration of the peak has also decreased in the last decade.

Temporal and spatial variability of Oithona spp. abundance and phenology in the North Atlantic, North Pacific and Southern Ocean using the CPR data

The genus Oithona is considered the most ubiquitous and abundant copepod group in the world oceans. Although they generally make-up a lower proportion of the total copepod biomass, because of their high numerical abundance, preferential feeding for microzooplankton and motile preys, Oithona spp. plays an important role in microbial food webs and can provide a food source for other planktonic organisms. Thus, changes in Oithona spp. overall abundance and the timing of their annual maximum (i.e. phenology) can have important consequences for both energy flow within marine food webs and secondary production. Using the long term data (1954-2005) collected by the Continuous Plankton Recorder (CPR), the present study, investigates whether global climate warming my have affected the long term trends in Oithona spp. population abundance and phenology in relation to biotic and abiotic variables and over a wide latitudinal range and diverse oceanographic regions in the Atlantic, Pacific and Southern Ocean.

Influence of the Pacific Decadal Oscillation on phytoplankton phenology and community structure in the western North Pacific

Phytoplankton phenology and community structure in the western North Pacific were investigated for 2001–2009, based on satellite ocean colour data and the Continuous Plankton Recorder survey. We estimated the timing of the spring bloom based on the cumulative sum satellite chlorophyll adata, and found that the Pacific Decadal Oscillation (PDO)-related interannual SST anomaly in spring significantly affected phytoplankton phenology. The bloom occurred either later or earlier in years of positive or negative PDO (indicating cold and warm conditions, respectively). Phytoplankton composition in the early summer varied depending on the magnitude of seasonal SST increases, rather than the SST value itself. Interannual variations in diatom abundance and the relative abundance of non-diatoms were positively correlated with SST increases for March–April and May–July, respectively, suggesting that mixed layer environmental factors, such as light availability and nutrient stoichiometry, determine shifts in phytoplankton community structure. Our study emphasised the importance of the interannual variation in climate-induced warm–cool cycles as one of the key mechanisms linking climatic forcing and lower trophic level ecosystems.

Volcanic ash fuels anomalous plankton bloom in subarctic northeast Pacific

Using multiple lines of evidence, we demonstrate that volcanic ash deposition in August 2008 initiated one of the largest phytoplankton blooms observed in the subarctic North Pacific. Unusually widespread transport from a volcanic eruption in the Aleutian Islands, Alaska deposited ash over much of the subarctic NE Pacific, followed by large increases in satellite chlorophyll. Surface ocean pCO2, pH, and fluorescence reveal that the bloom started a few days after ashfall. Ship-based measurements showed increased dominance by diatoms. This evidence points toward fertilization of this normally iron-limited region by ash, a relatively new mechanism proposed for iron supply to the ocean. The observations do not support other possible mechanisms. Extrapolation of the pCO2 data to the area of the bloom suggests a modest ∼0.01 Pg carbon export from this event, implying that even large-scale iron fertilization at an optimum time of year is not very efficient at sequestering atmospheric CO2.