In order to establish the potential role of Antarctic krill ( Euphausia superba ) in the recycling of bioactive elements, we have quantified the release of iron, phosphate, and ammonia by these organisms along the Antarctic Peninsula sector of the Southern Ocean. The experimental results suggested that the presence of krill has a significant impact on ambient iron concentrations, as large amounts of this trace element were released by the krill (22–689 nmol Fe g Dry Weight −1 h −1 , equivalent to 0.2 to 4.3 nmol Fe L −1 d −1 ). Half of this iron release occurred within the first hour of the experiment, and differences in iron and phosphate release rates (3.1 to 14.0 μ mol PO 4 3− g DW −1 h −1 ) seemed to reflect differences in food availability. These results identify krill as a major node in iron cycling in the Southern Ocean, potentially influencing iron residence time in the upper water column of this region.
Abstract Rapid warming of the Mediterranean Sea threatens marine biodiversity, particularly key ecosystems already stressed by other impacts such as Posidonia oceanica meadows. A 6‐year monitoring of seawater temperature and annual P. oceanica shoot demography at Cabrera Archipelago National Park (Balearic Islands, Western Mediterranean) allowed us to determine if warming influenced shoot mortality and recruitment rates of seagrasses growing in relative pristine environments. The average annual maximum temperature for 2002–2006 was 1 °C above temperatures recorded in 1988–1999 (26.6 °C), two heat waves impacted the region (with seawater warming up to 28.83 °C in 2003 and to 28.54 °C in 2006) and the cumulative temperature anomaly, above the 1988–1999 mean annual maximum temperature, during the growing season (i.e. degree‐days) ranged between 0 °C in 2002 and 70 °C in 2003. Median annual P. oceanica shoot mortality rates varied from 0.067 year −1 in 2002 to 0.123 year −1 in 2003, and exceeded recruitment rates in all stations and years except in shallow stations for year 2004. Interannual fluctuations in shoot recruitment were independent of seawater warming ( P >0.05). P. oceanica meadows experienced a decline throughout the study period at an average rate of −0.050±0.020 year −1 . Interannual variability in P. oceanica shoot mortality was coupled ( R 2 >0.40) to seawater warming variability and increasing water depth: shoot mortality rates increased by 0.022 year −1 (i.e. an additional 2% year −1 ) for each additional degree of annual maximum temperature and by 0.001 year −1 (i.e. 0.1% year −1 ) for each accumulated degree water temperature remained above 26.6 °C during the growing season. These results demonstrate that P. oceanica meadows are highly vulnerable to warming, which can induce steep declines in shoot abundance as well indicating that climate change poses a significant threat to this important habitat.
An extensive compilation of data for 96 phytoplankton species, 46 macroalgal species, 27 seagrass species, 11 species of freshwater angiosperms, and several mixed phytoplankton and macroalgal communities revealed a tendency toward higher concentrations of N and P in phytoplankton compared to those of macrophytes. The depletion of P, and to a lesser extent N, in macrophytes, particularly macroalgae, appears to reflect a greater degree of P and N limitation of growth of natural macrophyte populations, rather than an intrinsic difference in their chemical composition relative to that of phytoplankton. Close associations between nutrients, particularly a strong linear relationship between concentrations of N and P, reflect the similar biochemical basis of the different aquatic plant groups and appear to represent a fundamental characteristic of the plant kingdom. The results obtained indicate, therefore, that aquatic plants form a continuum across a unique pattern of change in nutrient concentrations, despite considerable differences in their architectural, evolutionary, and life histories, and the growth conditions encountered in their habitats.
