Phytoplankton preliminary results

What is phytoplankton?

Phytoplankton is the collection of microscopic photosynthetic cells that grow in the upper regions of the ocean, where sunlight is plentiful. These small algae are the base of the thophic chain and consequently its abundance determines the productivity of the marine trophic ecosystems, from small herbivores to fishes. The vertical distribution of the phytoplankton in the water column is restricted to the euphotic zone (the layer where incident light is enough to perform the photosynthesis). Therefore, its growth is mainly controlled by the availability of nutrients in this upper layer. The nutrient concentration in the surface ocean is mainly regulated by the water masses movement that transports nutrients from the deeper layers towards the euphotic layer. On general, these displacements are connected to the circulation patterns on the water masses in the ocean and wind regimen. The climate change could be modifying these circulation patterns and consequently could be affecting the nutrient availability for phytoplankton growth. Furthermore, in the coastal areas the discharges from land sources are also modifying the nutrient concentrations. Over geological time scale, the ocean has become the primary storage sink for atmospheric carbon dioxide. About 90 percent of the total carbon content in our Planet has settled to the bottom of the ocean, primarily in the form of dead biomass. Phytoplankton plays a central role is this process as photosynthesis photosynthesis provides more than 99% of the organic matter used by the marine food webs. Therefore, understanding the factors regulating the growth of the phytoplankton communities is necessary to predict the effects of the anthropogenic changes on the capacity of the ocean to capture atmospheric CO2.

How was the phytoplankton analyzed during NITROALBORAN project?

During May 2008 Nitroalboran cruise was carried out on board of the oceanographic vessel “García del Cid”. Figure 1 show where the stations were positioned.

Figure 1. Western Mediterranean Sea and Alborán Sea area. Soft blue color corresponds to downwelling area of the Werstern Anticyclone Gyre.

Water samples were collected at different depths in each station using a Rossete equipped with Niskin bottles. Samples were fixed with a solution of Lugol and preserved in a dark-cool place until analysis in laboratory.

Then, sub-samples of 25-50 ml were allowed to settle for 24 hours on Utermöhl chambers and finally observed by inverted light microscopy. Abundance and composition of phytoplankton was determined at genera or species level.

Some results of the analyses

In the study area, the phytoplankton abundance varied according to the nutrient concentration. Thus, two areas were clearly differentiated: the coastal zone (with a high nutrient concentration due to the occurrence of upwelling events that transported deep water enriched in nutrients towards the euphotic layer) and the oceanic stations where nutrients in the surface layer were scarce. Concordantly, the phytoplankton abundance was higher in the coastal areas than in oceanic ones, reaching values of 1590 cell ml-1 in station B2 and 1130 cell ml-1 in station A1. The abundances of phytoplankton in the oceanic stations (A5, B5) ranged between 200 an 214 cell ml-1 in A5 and B5, respectively.

Total phytoplankton abundance (Nitroalboran 0508). Sup= surface sample. MSF= depth of maximum fluorescence

Furthermore, the taxonomic composition of the communities varied widely. Thus, in the coastal stations, phytoplankton was dominated by diatoms while dinoflagellates and small flagellates were more relevant in oceanic waters.

Relative abundance of the mayor groups of phytoplankton (Nitroalboran 0508).

The most abundant diatoms were Chaetoceros sp., Leptocylindrus danicus, Leptocylindrus minimus, Pseudinotzschia sp., Asterionellopsis glacialis and Skeletonema costatum in coastal stations and Chaetoceros sp., Leptocylindrus danicus, Leptocylindrus minimus, Nistzschia sp, Rhizosolenia sp. and Guinardia delicatula in oceanic areas.

If you want to know more about the phytoplankton at the Alborán Sea read this article:

Seasonal an inter-annual variability of the phytoplankton communities in an upwelling area of the Alborán Sea (SW Mediterranean Sea). Mercado et al. 2005. Scientia Marina. 69 (4) 451-465.

http://scientiamarina.revistas.csic.es/index.php/scientiamarina/article/view/274/274


You can watch some pictures of the main phytoplankton species at the Alboran sea in the next link: http://www.ma.ieo.es/nitroalboran/gallery.html