Role of krill versus bottom-up factors in controlling phytoplankton biomass in the northern Antarctic waters of South Georgia

Abstract

The extent to which Antarctic phytoplankton stocks are controlled by 'bottom-up' and/or 'top-down' factors is highly variable. Here we consider data collected at South Georgia during 3 summer surveys that recorded substantial hydrographic variability. A suite of bottom-up and top-down controlling factors were measured simultaneously at the mesoscale. Sea surface temperature varied by > 2°C, macronutrients ranged from near-winter concentrations to near-depleted, while mean densities of a major grazer, krill Euphausia superba, varied between near-zero and > 400 g wet mass m -2 . A general linear model was used to identify the main factors implicated in the observed differences in phytoplankton biomass. Despite east-to-west and on-to off-shelf temperature gradients, temperature per se was not implicated in phytoplankton variability. Also, while there was an abundance of NO 3 -N in surface waters, NH 4 -N was the key nutrient throughout. A domed relationship between phytoplankton and krill peaked between 2 and 4 mg chlorophyll a m -3 and 6 and 30 g krill m -2 . The positive side of this dome was represented by the west off-shelf region downstream of South Georgia. Here, an ample supply of micro-and macronutrients promoted high primary production, and low densities of krill presumably had little grazing effect. This positive relationship between krill and phytoplankton biomasses was interpreted as krill accumulating in areas of good feeding conditions. The negative side of the dome was typified by the east off-shelf region, where macronutrients remained high, primary production rates were low, and krill densities were very high. The grazing rates calculated here suggested that krill affect their food stocks severely, and the negative krill-phytoplankton relationship in this region may reflect locally high krill densities driving down their food supply.

Distribution and recurrence of phytoplankton blooms around South Georgia, Southern Ocean

“…This is not surprising given the various independent factors that can influence surface Chl a concentrations. Among these: mixed layer depth and its effects on bloom dilution (Smetacek and Naqvi, 2008), dust input (e.g., Gassó et al, 2010), composition of the bloom (diatoms versus flagellates) and grazing (Whitehouse et al, 2009). Variations in flow intensity and pathways of the ACC fronts around South Georgia (Thorpe et al, 2002;Park et al, 2010;Boehme et al, 2008) are also among the most important factors likely influencing the interannual variability of the bloom: circulation around and then downstream of shallow topographic features is believed to control the magnitude of sediment-derived iron input to the water column (i.e., due to re-suspension processes) as well as its transport to more distant regions de Jong et al, 2012;Nishioka et al, 2011;Planquette et al, 2007).…”

Impact of climate change on Antarctic krill

“…Trivelpiece et al 2011). Furthermore, evidence is increasing that krill fulfil complex roles in ecosystem feedback loops through grazing and nutrient recycling (Tovar-Sanchez et al 2007, Whitehouse et al 2009, Schmidt et al 2012. Because Antarctic krill populations and marine ecosystems are responding to climate change, resource and conservation management in the Southern Ocean will need to become much more adaptive.…”

Sedimentary and atmospheric sources of iron around South Georgia, Southern Ocean: a modelling perspective

“…Weaker winds reduce mixing processes but also generate a microclimate with greater sunshine and higher local air temperatures (Brandon et al, 1999) that will locally modify the properties and vertical structure of the water column. Moreover, Brandon et al, 1999 andWhitehouse et al, 2009 indicated that the eastern shelf region of the island is rich in biological activity, therefore topdown controls by macrozooplankton like krill, not included in the model (Fig. 3) could also locally control phytoplankton biomass.…”