Phytoplankton biomass and temperature trends in offshore waters of the Basque country

GLOBEC INTERNATIONAL NEWSLETTER APRIL 2010 Phytoplankton biomass and temperature trends in offshore waters of the Basque country Marta Revilla, Ángel Borja, Almudena Fontán, Javier Franco, Manuel González, Victoriano Valencia and Luis Ferrer Marine Research Division, AZTI-Tecnalia, Pasaia, Gipuzkoa, Spain Temporal trends (minimum, central and maximum values) of surface chlorophyll a and SST are shown in Figure 2. Here, values are the difference between the measured data and the data estimated for an average year. These latter are calculated by statistical itting, using the monthly mean values and assuming that the variable analysed can be deined as a linear combination of basis functions (Revilla et al., 2010). In spite of the irregular sampling, a signiicant increasing trend in the SST has been identiied (1.35 · 10 -2 ± 1.07 · 10 -2 °C · year -1). This trend is very similar to that observed at a neighbouring inshore station (Aquarium, Donostia-San Sebastián, see Figure 1), for which daily records of temperature are available. At the offshore station, the surface chlorophyll a presents a slight decreasing trend (-3.8 · 10 -3 ± 4.7 · 10-3 μg · l -1 · year -1) over the past two decades, which is also signiicant statistically. Although these results support the hypothesis that phytoplankton biomass is responding to changes in the water temperature within the southern Bay of Biscay, the response could be very slight (at least in surface waters). Further research is required to determine multi-decadal trends of chlorophyll a through the water column. Scale (km) 0 2 25 m 2.5 2.0 Data Central trend Minimum trend Maximum trend 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 3.0 b) 2.5 2.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5 -3.0 Figure 2. Temporal trends of: a) surface chlorophyll a; and b) sea surface temperature, at station D2, from 1986 to 2008. Values are the difference between the measured data and the data estimated for an average year. Acknowledgements The Department of Agriculture, Fisheries and Food of the Basque Government has funded this study through the project “Variaciones”, which includes acquisition of the in situ data. Part of this study has been funded by the project “K-Egokitzen” from the Department of Industry, Trade and Tourism of the Basque Government. We are grateful to Luis Cuesta from the Marine Research Division of AZTI-Tecnalia, for his help on CTD data acquisition. We also thank Professor Michael Collins for advising us on this contribution. m -1 -75 m -50 m a) D2 4 -1 00 3.0 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 Seasonal and interannual variability of the surface chlorophyll a and sea surface temperature (SST) at an offshore station in the southeastern Bay of Biscay (named D2 in Figure 1) were analysed. The main objective of this analysis is to look for long-term trends in chlorophyll a concentration, as a proxy for phytoplankton biomass, in relation to the changes observed in the SST in the Basque coastal waters. For the period 1986–2008, physico -chemical and chlorophyll a data were obtained annually, by means of CTD proiles and bottle samples at the location 43°27’N, 01°55’W. This offshore station is considered not to be impacted by anthropogenic inluence, due to its distance (13 km) from the main pollution sources on land; as such, this makes it suitable for exploring the response of phytoplankton to oceano - meteorological forcing. The surveyed months and the total number of surveys per year have been very variable (from 2 to 12). m -25 Aquarium San Sebastián Reference Figure 1. Map showing the location of the sampling station, D2 (43°27’N, 01º55’W), within the context of the Bay of Biscay. The neighbouring station, Aquarium (43º19’N, 02°00’W), is also shown. Revilla M., A. Borja, A. Fontán, J. Franco, M. González and V. Valencia. 2010. A two-decade record of surface chlorophyll a and temperature in offshore waters of the Basque country (southeastern Bay of Biscay). Revista de Investigación Marina 17(2): 13 – 20. 12