Primary production throughout austral fall, during a time of decreasing daylength in the western Antarctic Peninsula

Abstract

Antarctic phytoplankton is characterized by a pronounced seasonality in abundance, driven mainly by changes in sunlight. We combined measurements and modeling to describe the influence of changing daylength on fall and winter phytoplankton production in coastal waters of the western Antarctic Peninsula (wAP) in 2001 and 2002. The model was parameterized with field observations from the Palmer Long-Term Ecological program in the wAP during summer and early fall and from the Southern Ocean Global Ecosystems Dynamics program fall and winter cruises to Marguerite Bay and shelf waters. Shorter daylength and a deepening of the mixed layer account for most of the decrease in primary production during March, April, and May. At this time, biomass decreases by an order of magnitude and remains low and constant until the end of August. An additional loss rate was added to the primary production model to fit output to observations. This loss rate, estimated at ~0.1 to 0.15 d −1 , is due to physical, chemical, and biological processes such as scavenging by sea ice, zooplankton grazing, cell lysis, and cell sedimentation, which are expected to be high at this time of year. Growth and loss rates of phytoplankton populations are similar on 1 March, with growth decreasing rapidly over time while the loss rates remain constant. By the beginning of winter (1 June), growth is low, with minimum rates in July and increasing towards September. During a period of diminishing food supply, preliminary estimates of grazing indicate that fall biomass could support existing macrozooplankton populations, but the timing and concentration of food supply is variable and expected to affect health of zooplankton as they enter the winter.KEY WORDS: Phytoplankton · Primary production · Antarctica · Bloom demise · Fall · Winter · Growth rates · Loss rates Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 452: [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] 2012 summer of 1 to 3 mg chlorophyll a (chl a) m −3 with peaks or blooms up to 30 mg m −3 (Smith et al. 1998) decreases to a low of < 0.2 mg m −3 in the winter , Fritsen et al. 2008, Meyer et al. 2009). In the fall, phytoplankton biomass is variable, with chl a concentrations anywhere from 0.1 to 2 mg m −3 within the surface mixed layer (Vernet et al. 2011).Our knowledge of fall and winter phytoplankton dynamics in the Southern Ocean is limited by the difficulty of sampling underneath the ice and the lack of ocean color remote sensing images from April to October due to low sun angle (Marrari et al. 2008). This lack of information precludes understanding variability due to latitude and interannual changes in the water column. Scarce supply of food during winter combined with low starvation tolerance of some zooplankton, e.g. larval krill, might necessitate their use of an alternative food source (Walsh et al. 2001). Several investigators (Daly 1990, Smetacek et al. 1990, Quetin & Ross 1991 have suggested that sea i...

Net primary productivity estimates and environmental variables in the Arctic Ocean: An assessment of coupled physical-biogeochemical models

“…Some Arctic phytoplankton species are shadeacclimated, especially in spring, though most available data are for summer, lightadjusted phytoplankton [Sakshaug, 2004;Harrison et al, 1982]. Arctic phytoplankton community structure appears to change toward smaller cells in response to warming both in the field and in experiments [Li et al, 2009;Tremblay et al, 2009;CoelloCamba et al, 2014CoelloCamba et al, , 2015 that may have different nutrient uptake kinetics [Lovejoy et al, 2007], while individual phytoplankton species may have narrower temperature ranges for optimal growth, usually in combination with light requirements [Harrison and Platt, 1986;Vernet et al, 2012]. These phenological changes will require the inclusion of multiple phytoplankton size classes in models and/or different physiological rates.…”

Physiological response of adult Antarctic krill, Euphausia superba, to long-term starvation

“…Winter season in the Antarctic region is characterised by reduced day length and low light intensity and, as a result, scarcity of algal food (Atkinson et al 2002;Vernet et al 2012). There is very little information, however, that provides an explanation as to what extent either environmental conditions contribute to the behavioural and physiological changes that take place in overwintering krill.…”

“…For most of the krill population, almost complete darkness in winter alternates with near constant availability of daylight in summer. This, in turn, means almost no algal food in the water column in winter with\0.1 lg L -1 Chl a and primary production lower than 1 mg Cm -2 day -1 , in contrast with massive phytoplankton blooms of [1,000 mg Cm -2 day -1 in spring and early summer, resulting in Chl a concentrations of more than 10 lg L -1 (Atkinson et al 2002;Vernet et al 2012). In addition, the decline in algal food availability during autumn can be very rapid (Vernet et al 2012).…”

Assessing Drivers of Coastal Primary Production in Northern Marguerite Bay, Antarctica

“…The coastal regions and Marguerite Bay were most productive, reaching maxima of ∼5,000-7,200 mg C m −2 d −1 in highly productive years (Ducklow et al, 2012b;Weston et al, 2013;Stukel et al, 2015). A study modeling weekly PP measurements used a simple, but often used, model which fits their data reasonably and leaving room for improvement (Behrenfeld and Falkowski, 1997;Vernet et al, 2012). This model did not take the composition of the phytoplankton or macro nutrient availability into account, parameters previously known to be affected by the observed changes in climate.…”

“…Our main objective is to reassess the relations between PP, phytoplankton biomass, and species composition, light, macronutrient concentrations, and water column properties. Thereby, we study if parameters not traditionally used to model PP, such as variability in phytoplankton species composition and macro nutrient concentrations, would need to be included given rapid changes currently unfolding in the WAP and improve current models (Behrenfeld and Falkowski, 1997;Vernet et al, 2012). Most previously used models are appropriate for open ocean applications and not necessarily coastal systems.…”