CHAPTER I. INTRODUCTORY RIVERS, FRESH-WATER AND TIDAL FEATURES COMMON TO BOTH — THEIR RESPECTIVE POWERS — DIFFERENCE OF EXISTING OPINION AS TO THEIR RELATIVE IMPORTANCE — THE CAPACITY OF THE OUTLETS OF TIDAL RIVERS CAN ONLY BE MAINTAINED BY STRICT CONSERVATION OF THE TIDAL QUANTITY — THE DOCTRINE SUPPORTED BY PROFESSIONAL OPINIONS AND WELL-KNOWN CASES OF HARBOUR DECAY.

Abstract. Antarctica's Getz Ice Shelf has been rapidly thinning in recent years, producing more meltwater than any other ice shelf in the world. The influx of fresh water is known to substantially influence ocean circulation and biological productivity, but relatively little is known about the factors controlling basal melt rate or how basal melt is spatially distributed beneath the ice shelf. Also unknown is the relative importance of subglacial discharge from the grounded ice sheet in contributing to the export of fresh water from the ice shelf cavity. Here we compare the observed spatial distribution of basal melt rate to a new sub-ice-shelf bathymetry map inferred from airborne gravity surveys and to locations of subglacial discharge from the grounded ice sheet. We find that melt rates are high where bathymetric troughs provide a pathway for warm Circumpolar Deep Water to enter the ice shelf cavity and that melting is enhanced where subglacial discharge fresh water flows across the grounding line. This is the first study to address the relative importance of meltwater production of the Getz Ice Shelf from both ocean and subglacial sources.

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Kinetics of ATP- and Na(+)-gradient driven Ca2+ transport in basolateral membranes from gills of freshwater- and seawater-adapted tilapia

Journal of Experimental Biology ◽

10.1242/jeb.186.1.95 ◽

1994 ◽

Vol 186 (1) ◽

pp. 95-108 ◽

Cited By ~ 4

Author(s):

P. M. Verbost ◽

T. J. Schoenmakers ◽

G. Flik ◽

S. E. Wendelaar Bonga

Keyword(s):

Fresh Water ◽

Plasma Membranes ◽

Sea Water ◽

Basolateral Membrane ◽

Relative Importance ◽

Ambient Conditions ◽

Sodium Homeostasis ◽

Kinetics Of ◽

Exchange Activity

Plasma membranes of the gills of freshwater- and seawater-adapted tilapia were analyzed for Ca(2+)-ATPase and Na+/Ca2+ exchange activity. The relative importance of ATP-driven and Na(+)-gradient-driven Ca2+ transport in Ca2+ extrusion was evaluated on the basis of kinetic analyses in vitro. The Na+/Ca2+ exchangers in branchial membranes from freshwater or seawater fish displayed similar kinetics. The ATP-driven Ca2+ pump, however, showed a somewhat lower affinity for Ca2+ in membranes isolated from seawater gills than in membranes from freshwater gills; no difference in Vmax was found. The activity of the exchanger was estimated to be 50% of that of the ATP-driven pump at prevailing cytosolic Ca2+ concentrations (10(−7) mol l-1). Opercular ionocyte densities and branchial Na+/K(+)-ATPase content were not significantly different in fish residing in fresh water or sea water. We conclude that the gills of tilapia living for prolonged periods in fresh water or sea water do not differ in the make-up of their basolateral membrane with regard to Ca(2+)-ATPase, Na+/Ca2+ exchange and Na+/K(+)-ATPase activity. Apparently, the densities of these carriers suffice for calcium and sodium homeostasis under these vastly different ambient conditions.

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