PRIMARY PRODUCTIVITY I

ON THE USE OF OPTICAL MEASUREMENTS TO CONSTRAIN MODELS OF MARINE PRIMARY PRODUCTIVITY AND PARTICLE DYNAMICS

Cullen, John¹; Comeau, Adam¹; Craig, Susanne E.¹; Davis, Richard F.¹; Fennel, Katja<sup>1

1</sup>Dalhousie University Department of Oceanography, Halifax, NS, B3H 4J1, Canada

Just over 50 years ago, models were developed to describe photosynthesis in the water column as a function of chlorophyll concentration, irradiance at the surface, the penetration of photosynthetically available radiation (PAR), and parameters of the relationship between photosynthesis and PAR. At the time there was not much interest in relating estimated primary production normalized to chlorophyll directly to growth rates of phytoplankton -- the two are linked through the ratio of cellular carbon to chlorophyll (C:Chl). Estimation of growth rates is central to most models of marine ecosystem dynamics, and comparison with observations (e.g., fields of chlorophyll concentration estimated from remote sensing) is necessary for validation. Consequently, specification of C:Chl has become increasingly important in marine ecosystem modelling. To support this, optical assessment of phytoplankton carbon has recently been incorporated into remote-sensing models of primary production and growth rates of phytoplankton. Meanwhile, a number of optically-based approaches have been developed for assessing physiological properties of phytoplankton. Most use measurements of chlorophyll fluorescence: either sun-induced or stimulated by in situ fluorometers. We describe an approach to estimate the saturation parameter for photosynthesis from surveys using conventional fluorometers. We also examine possible links between sun-induced fluorescence yield and variability in the maximum rate of photosynthesis, where photosynthesis is normalized to light absorption rather than to chlorophyll, which we argue is an outdated variable in bio-optics. We are now in a position to consider a new class of phytoplankton model, in which most, if not all terms -- available and absorbed spectral radiation, photosynthetic parameters, biomass of phytoplankton -- can be directly constrained by optical measurements, which in turn can be incorporated into assimilative models of primary productivity and particle dynamics. Development and validation of such models will require old-fashioned measurements from ships to complement advanced optical sampling, laboratory studies and new approaches to modelling.

PRIMARY PRODUCTION IN THE SUBTROPICAL CONVERGENCE ZONE: COMPARISON OF IN-SITU MEASUREMENT METHODS AND REMOTE ESTIMATES

Schwarz, Jill¹; Gall, Mark²; Pinkerton, Matt¹; Kennan, Sean<sup>1

1</sup>National Institute for Water & Atmospheric Research NIWA, Private Bag 14901, Kilbirnie, Wellington, --, 6031, New Zealand; ²NIWA, PO Box 8602 Riccarton, Christchurch, Canterbury, 8011, New Zealand

The Subtropical Front (STF) is a 25,000 km-long convergence zone of subtropical and subantarctic waters that encircles the globe at around 45 degrees south. High phytoplankton abundance in the STF to the east of New Zealand is a conspicuous feature of ocean colour imagery, attributable to the front being bathymetrically locked to a broad submarine ridge extending eastwards from the New Zealand landmass.

The current understanding of optical, hydrodynamic, and biogeochemical factors setting the conditions for phytoplankton productivity in this region are summarised. A decade of ocean colour measurements were analysed by Objective Analysis to describe the seasonal climatology, time and space scales of covariance, and dominant modes of variability in the region. Much of the variability in the ocean colour climatology of the STF is explained by the seasonal and semiannual cycles (major austral spring and minor austral fall bloom), but there remains a large portion of unexplained variance suggesting that mesoscale eddies and transient fronts are important.

A range of at-sea approaches for estimating primary production in the STF over Chatham Rise are compared. Measurements include size-fractionated carbon-14 incubations, sun-induced fluorescence profiles, above-surface water-leaving radiance measurements, absorption-based estimates, and finally satellite-derived values from the MODIS fluorescence line-height evaluation product and the Vertical Generalized Production Model (VGPM). The compatibility and comparability of the different measurement techniques is presented. Knowledge of the physiological parameters PBmax and alphaK remains the major constraint on accurate derivation of daily production values from remote measurements.

THE INFLUENCE OF SEA ICE ON ARCTIC OCEAN MARINE PRIMARY PRODUCTION: REGIONAL RESPONSES.

Hill, Victoria ¹; Matrai, Patricia²; Olsen, Elise²; Zimmerman, Richard<sup>1

1</sup>Old Dominion University 4600 Elkhorn Ave, Norfolk, VA, 23529, United States; ²Bigelow Laboratory for Ocean Sciences, W. Boothbay Harbor, ME, 04575-475, United States

Summer sea ice cover in the Arctic Ocean has been declining rapidly in recent years, this is expected to have significant impact on marine primary production (PP) through changes in irradiance and stratification. Primary production was estimated using a vertically integrated model based on satellite retrieved chlorophyll a and euphotic depth. This model was applied to the current SeaWiFs data record (1997-2007). The Arctic Ocean was divided into functional regions classified by the dominant physical – biological forcing, these were (a) inflow shelves, (b) interior shelves, (c) outflow shelves and (d) deep central basin. Patterns and trends in PP within each of these regions was then analyzed in tandem with ice concentration data from passive microwave sensors (SSM/I). An increasing trend in total annual PP has been observed in the last five years (2003-07), with a corresponding decrease in areal specific PP. This is attributed to the increasing ice retreat over the central basin, although increasing the total area for PP low nutrient concentrations here limits PP, thereby reducing the overall area specific PP. Regional responses to the reduction in summer sea ice have differed, for example in areas which have long been mostly ice free in the summer (Barents Sea, inflow shelf) the small inter-annual variations in open water do not have a strong linear correlation with PP. Other factors are influencing the magnitude and seasonality of PP in this region, possibly the strong advective influence from the south. In areas which were always fully or mostly ice covered in the past (central basin, interior shelves), the successive years of reduced sea ice cover have been a major driver in increasing PP. As the Arctic ocean continues to open up PP will become more strongly influenced by inter-annual variation in solar induced stratification, freshwater inflow and wind driven mixing.