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Vernet, M, Sines K, Chakos D, Cefarelli AO, Ekern L.  2011.  Impacts on phytoplankton dynamics by free-drifting icebergs in the NW Weddell Sea. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 58:1422-1435.   10.1016/j.dsr2.2010.11.022   AbstractWebsite

Glacier ice released to the oceans through iceberg formation has a complex effect on the surrounding ocean waters. We hypothesized that phytoplankton communities would differ in abundance, composition and production around or close to an iceberg. This paper tests the influence of individual icebergs on scales of meters to kilometers, observed through shipboard oceanographic sampling on March-April 2009. Surface waters (integrated 0-100 m depth, within the euphotic zone) sampled close to the iceberg C-18a ( <1 km) were characterized by lower temperatures, more dissolved nitrate, less total chlorophyll a (chla) concentration, less picoplankton ( <3 mu m) cell abundance, and higher transparency than surface conditions 18 km upstream. However, enrichment of large cells, identified as diatoms, was the basis of an active food chain. Upward velocity of meltwater and dissolved Fe concentrations in excess of 1-2 nM are expected to facilitate diatom specific growth. The presence of diatoms close to the iceberg C-18a and the higher variable fluorescence (Fv/Fm) indicated healthy cells, consistent with Antarctic waters rich in micronutrients. Furthermore, chla increased significantly 2 km around the iceberg and 10 days after the iceberg's passage. We hypothesize that the lower biomass next to the iceberg was due to high loss rates. Underwater melting is expected to dilute phytoplankton near the iceberg by entraining deep water or by introducing meltwater. In addition, high zooplankton biomass within 2 km of the iceberg, mainly Antarctic krill Euphausia superba and salps Salpa thompsonii, are expected to exert heavy grazing pressure on phytoplankton, the krill on large cells >10 mu m and the salps on smaller cells, 3-10 mu m. The iceberg's main influence in the austral fall is measured not so much by phytoplankton accumulation but by reactivation of the classic Antarctic food chain, facilitating diatom growth and sustaining high Antarctic krill populations. (C) 2011 Elsevier Ltd. All rights reserved.

Whitehead, K, Vernet M.  2000.  Influence of mycosporine-like amino acids (MAAs) on UV absorption by particulate and dissolved organic matter in La Jolla Bay. Limnology and Oceanography. 45:1788-1796.   10.4319/lo.2000.45.8.1788   AbstractWebsite

Experimental work with cultures of the red tide dinoflagellate Lingulodinium polyedrum suggested mycosporine-like amino acids (MAAs) are a component of the organic matter excreted by cells. MAAs in dissolved organic matter (DOM) also may have it large influence on absorption of ultraviolet (UV) light through the water column. To test these hypotheses in a natural setting, data were collected from March 1995 through April 1996 in coastal waters off California, U.S.A. During this time, a large red tide of L. polyedrum occurred in March and April 1995. In this field study, we show that MAAs are a quantifiable component of DOM, particularly during the period following the red tide events. Absorption spectra of particulate organic matter (POM) and DOM samples revealed disproportionately high levels of UV absorption relative to visible wavelengths. UV absorption by POM at 330 nm was linearly related to phytoplankton biomass (between 0-10 mug chlorophyll [Chl] a L-1), measured as Chi a, accounting for 71% of the variability in absorption. Chromatographic analyses revealed the presence of various MAAs in both POM and DOM pools. MAAs were observed in 83% (n = 53) and 47% (n = 13) of the samples analyzed with concentrations ranging from 0 to 2.75 muM (0-696.00 mu mol mug(-1) Chi a) in the POM and 0 to 111.40 nM in the DOM fraction. Absorption by dissolved MAAs, as calculated from the measured concentrations, correlated with DOM UV absorption (r(2) = 0.77) and accounted for up to 10% of the total DOM absorption at 330 nm. Thus, MAAs are a small but quantifiable component of the DOM pool in the field and contribute to UV absorption.

Garibotti, IA, Vernet M, Smith RC, Ferrario ME.  2005.  Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula. Journal of Plankton Research. 27:825-843.   10.1093/plankt/fbi056   AbstractWebsite

The spatial distribution of phytoplankton cell abundance, carbon (C) biomass and chlorophyll a (Chl a) concentration was analysed during three summers (1996, 1997 and 1999) in a seasonal sea-ice area, west of the Antarctic Peninsula. The objective of the study was to assess interannual variability in phytoplankton spatial distribution and the mechanisms that regulate phytoplankton accumulation in the water column. Phytoplankton C biomass and Chl a distributions were consistent from year to year, exhibiting a negative on/offshore gradient. The variations in C concentration had a close and non-linear relationship with the upper mixed layer depth, suggesting that the vertical mixing of the water column is the main factor regulating phytoplankton stock. The magnitude of C gradients was 5-fold higher during 1996 than during 1997 and 1999. This was ascribed to interannual variations in the concentration of diatom blooms in the region influenced by sea-ice melting. Vertical distribution of the phytoplankton, as estimated from Chl a profiles, also varied along an on/offshore gradient: Chl a was distributed homogeneously in the upper mixed layer in coastal and mid-shelf stations and concentrated in the deep layer (40-100 m) occupied by the winter waters (WW, remnants of the Antarctic surface waters during summer) in more offshore stations. The region with a deep Chl a maximum layer (DCM layer) was dominated by a phytoplankton assemblage characterized by a relatively high concentration of diatoms. The extent of this region varied from year to year: it was restricted to pelagic waters during 1996, extended to the shelf slope during 1997 and occupied a major portion of the area during 1999. It is hypothesized that iron depletion in near surface waters due to phytoplankton consumption, and a higher concentration in WW, regulated this vertical phytoplankton distribution pattern. Furthermore, we postulate that year-to-year variations in the spatial distribution of the DCM layer were related to interannual variations in the timing of the sea-ice retreat. The similarity between our results and those reported in literature for other areas of the Southern Ocean allows us to suggest that the mechanisms proposed here as regulating phytoplankton stock in our area may be applicable elsewhere.

Vernet, M, Smith KL, Cefarelli AO, Helly JJ, Kaufmann RS, Lin H, Long DG, Murray AE, Robison BH, Ruhl HA, Shaw TJ, Sherman AD, Sprintall J, Stephenson GR, Stuart KM, Twining BS.  2012.  Islands of ice: influence of free-drifting Antarctic icebergs on pelagic marine ecosystems. Oceanography. 25:38-39.   10.5670/oceanog.2012.72   AbstractWebsite

Regional warming around West Antarctica, including the Antarctic Peninsula, is related to the retreat of glaciers that has resulted in significant ice mass loss in recent decades (De Angelis and Skvarca, 2003). Large icebergs (> 18.5 km long) originating from ice shelves in the Ross and Weddell Seas (Scambos et al., 2000) are attributed primarily to major loss events in these regions. Once free, icebergs become entrained in the counterclockwise Antarctic Coastal Current (Figure 1), eventually entering a strong northward flow in the Northwest Weddell Sea. We examined free-drifting icebergs in the Atlantic sector of the Southern Ocean in December 2005, aboard ARSV Laurence M. Gould, and in June 2008 and March/April 2009, aboard RVIB Nathaniel B. Palmer. Prior to these studies, little information was available about the effects of icebergs on the pelagic realm. On these cruises, we investigated the "iceberg ecosystem" (Smith et al., 2007; Smith, 2011) to assess the degree to which icebergs are (1) hotspots of biological activity across multiple trophic levels, and (2) focal points for enhanced export of organic carbon to the deep sea. An important focus of this work was to examine the fundamental mechanisms by which icebergs affect the pelagic ecosystem, including physical disruption and effects on the availability of critical nutrients (e.g., iron, nitrate).