In situ measurements
In order to assess the ocean optics especially in the open waters and identify directly the distribution of phytoplankton groups in these waters, we took in-situ water samples and direct optical measurements during several cruises in the last four years. Ground-truth data is essential for the retrieval of remote sensing data and the use of various methods to analyse phytoplankton composition will facilitate interpretation of results in future.
Past cruises
- RV Polarstern, ANT XXIV-1, from Bremerhaven (Germany) to Cape Town (South Africa) in Oct/Nov 2007
- RV Polarstern, ANT XXIV-4, from Punta Arenas (Chile) to Bremerhaven (Germany) in Apr/May 2008
- RV Maria S. Merian, MSM09-1, from Bremen (Germany) to St. John’s (Canada) in Jul/Aug 2008
- RV Polarstern, ANT XXV-1, from Bremerhaven (Germany) to Cape Town (South Africa) in Nov/Dec 2008
- RV Sonne, transit, from Kushiro (Japan) to Townsville (Australia) in Oct 2009
- RV Polarstern, ANT XXVI-4, from Punta Arenas (Chile) to Bremerhaven (Germany) in Apr/May 2010
- RV Polarstern, ARK XXV-1, Greenland Sea, from Bremerhaven (Germany) to Longyearbyen (Norway) in June 2010
- RV Poseidon, P399, from Mindelo (Cap Verde) via Las Palmas (Spain) to Vigo (Spain) in June 2010
- RV Polarstern, ARK XXV-2, Fram Strait, from Lonyearbyen (Norway) to Reykjavik (Iceland) in June/July 2010
- RV Polarstern, ARK XXVI-3, Transarc, from Tromsö (Norway) to Bremerhaven (Germany) Aug-Oct 2011
- RV Sonne, SHIVA-Sonne, from Singapore to Manila (Phillipines) in Nov 2011
Planned cruises
- RV Polarstern, ANT XXVIII-3, Eddy Pump, from Cape Town (South Africa) to Punta Arenas (Chile) in Jan-Mar 2012
Work at sea
The three ramses radiometers for underwater light measurements
1. Water samples
- Water samples are taken frequently from beneath the ship (moonpool) and – if possible - at the stations from CTD/rosette casts and processed for various analyses:
- Water samples are filtered onto GF/F filters for pigment analysis bei HPLC, particulate absorption measurements and POC.
- Water samples are preserved for flow cytometry measurements later in the laboratory in Bremerhaven.
- Particulate absorption in suspension and absorption of Gelbstoff are measured during the cruise using the point-source integrating-cavity absorption meter (PSICAM) (Röttgers et al., 2005).
2. Online and In Situ Optical Measurements
- A FastTracka (Chelsea) Fast Repetition Rate Fluorimeter (FRRF) is used in a flow-through system with water continuously pumped from the moonpool to provide online data of chlorophyll fluorescence during the cruise.
- Remote sensing reflectance is measured from onboard the ship with a set of three radiometers and in the water column (0-150 m) at the stations. The in-situ data are used to validate MERIS, MODIS and SeaWiFS remote sensing reflectance and Chl a products.
Results
The biooptical and biogeochemical data are used to construct and evaluate radiative transfer models and ocean color satellite retrievals (see respective projects). However, the in-situ results by themselves are also used to assess the phytoplankton dynamics and to develop strategies and techniques for automated in-situ measurements providing information on phytoplankton distribution and physiology where satellite data are not available (e.g. profile information, data under cloud cover, more detailed physiological information).
Bio-optical provinces in the Atlantic Ocean and their biogeographical relevance
Map of study area, showing the sample stations during cruise ANT-XXV/1 on RV Polarstern in November 2008 overlying the oceanic provinces defined by Longhurst (2006). The colours represent the pigment-based clusters defined by an unsupervised cluster algorithm (figure adapted from Taylor et al. 2011).
The relationship between phytoplankton assemblages and the associated optical properties of the water body is important for the further development of algorithms for large-scale remote sensing of phytoplankton biomass and the identification of phytoplankton functional types (PFTs), which are often representative for different biogeochemical export scenarios. Optical in-situ measurements aid in the identification of phytoplankton groups with differing pigment compositions and are widely used to validate remote sensing data. In this study we present results from an interdisciplinary cruise aboard the RV Polarstern along a north-to-south transect in the eastern Atlantic Ocean in November 2008. Phytoplankton community composition was identified using a broad set of in-situ measurements. Water samples from the surface and the depth of maximum chlorophyll concentration were analyzed by high performance liquid chromatography (HPLC), flow cytometry, spectrophotometry and microscopy. Simultaneously, the above- and underwater light field was measured by a set of high spectral resolution (hyperspectral) radiometers. An unsupervised cluster algorithm applied to the measured parameters allowed us to define bio-optical provinces, which we compared to ecological provinces proposed elsewhere in the literature. As could be expected, picophytoplankton was responsible for most of the variability of PFTs in the eastern Atlantic Ocean. Our bio-optical clusters agreed well with established provinces and thus can be used to classify areas of similar biogeography. This method has the potential to become an automated approach where satellite data could be used to identify shifting boundaries of established ecological provinces or to track exceptions from the rule to improve our understanding of the biogeochemical cycles in the ocean. For details of this work see Taylor et al. 2011.
Ongoing research
Current work focuses on applying the method on a lager data set, i.e. bio-optical in-situ data of all recent PHYTOOPTICS cruises in order to develop the best technique for assessing different biogeochemical provinces. Additionally the in-situ data from flow cytometry, HPLC and spectrofluorometry in the Atlantic and Pacific Oceans are being analyzed and compared to assess the phytoplankton group composition and discuss the advantages and drawbacks of each method. One main focus is given to the picoplankton which dominate mainly in the oligotrophic parts of the ocean.
References
Röttgers R., Schönfeld W., Kipp P.-R., Doerffer R. (2005). Practical test of a point-source integrating cavity absorption meter: the performance of different collector assemblies. Applied Optics 44(26): 5549-5560.
Taylor, B. B., Torrecilla, E., Bernhardt, A., Taylor, M. H., Peeken, I., Röttgers, R., Piera, J., and Bracher, A.: Bio-optical provinces in the Atlantic Ocean and their biogeographical relevance, Biogeosciences Discuss., 8, 7165-7219, doi:10.5194/bgd-8-7165-2011, 2011.
