M. Angelica Peña

Modelling the response of the planktonic food web to iron fertilization and warming in the NE subarctic Pacific

Abstract A one-dimensional ecosystem model with two explicit size classes of phytoplankton was developed for the NE subarctic Pacific to investigate variations in the export of organic particles to the ocean interior due to potential changes in the environment. Specifically, the responses of the planktonic ecosystem to permanent removal of iron limitation and to warming (of 2 and 5 °C) were explored. The ecosystem model consists of five components (small and large phytoplankton, microzooplankton, detritus and nitrogen), and includes grazing by mesozooplankton that varies in time according to long-term observations at Ocean Station Papa (OSP). The model addresses the role of iron limitation on phytoplankton growth and includes temperature dependence of physiological rates. The ecosystem model was forced with annual wind and solar heating from OSP. The model best reproduced the low chlorophyll high nitrate conditions of the NE subarctic Pacific when both small and large phytoplankton were limited by iron such that their maximum specific growth rate was reduced by 10 and 70%, respectively. Sensitivity analysis showed that model results depended on the value of the iron limitation parameter of large phytoplankton ( L Fe-L ) and the grazing parameters of micro- and mesozooplankton. To explore the effect of iron limitation, simulations were carried out varying the iron limitation parameters while maintaining the nitrogen flux at the base of the model constant and the grazing pressure by mesozooplankton unchanged. In the warming case, simulations were carried out increasing ocean temperatures by 2° and 5 °C applied only to the ecological components, the flux of nitrate at the base of the model was increased to obtain a steady annual cycle, and grazing by mesozooplankton remained constant. When compared with the standard case, model simulations indicated that both permanent removal of iron limitation and warming cause changes in food web structure and the carbon cycle. The response was more dramatic in the iron-replete case where the phytoplankton community structure in spring changed from one dominated by pico- and nanoplankton to one dominated by large phytoplankton, and primary production increased until it consumed all the external nutrient ( N ) supply to the upper layer. However, reducing iron deficiency actually led to lower annual primary production due to a decrease in the regeneration of nitrogen in the euphotic zone. These changes in food web structure influenced the magnitude, composition and seasonal cycle of sinking particles.

Decadal Trends in Oxygen Concentration in Subsurface Waters of the Northeast Pacific Ocean

Abstract Previous studies have shown decreasing oxygen concentration (O2) in subsurface waters of the continental slope from California to Canada since about 1980. With longer time series we show that from southern California to northern Canada increasing O2 preceded these decreases from 1950 to about 1980. Because there has been no clear trend since 1950, we cannot yet conclude that anthropogenic climate change is the cause of these decreasing trends after 1980. These findings are based mainly on O2 on the 26.7 potential density (σθ) surface in the region north of 30°N and east of 170°W, covering both the continental margin and deep-sea regions. On the continental slope, O2 increased at most locations by 10 to 20 µmol kg−1 to about 1980, followed by declines of similar magnitude in recent years. Changes in O2 were associated with changes in temperature of the opposite sign south of 37°N, but correlation of temperature and O2 is irregular in more northerly locations. At all locations, temperature-related solubility change was a minor cause of these O2 trends. In deep-sea waters, O2 decreased with time with a more rapid decrease from about 1995 to about 2003. At Ocean Station P (OSP; 50°N, 145°W), which has the longest uninterrupted record of observations, significant linear trends of −0.4 to −0.5 µmol kg−1 y−1 were found on the 26.5, 26.7, and 26.9 σθ surfaces. In addition, a significant sinusoidal oscillation of period 18.61 years and amplitude of 18 µmol kg−1 was found on the 26.9 σθ surface at OSP and a station 400 km to the east, which fits reasonably well with the lunar nodal cycle. The phase of this oscillation was identical at both locations. Clear evidence of similar variability did not emerge at other open-ocean locations or along the continental slope.