Phytoplankton Primary Production
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16 Phytoplankton Primary Production Margareth Kyewalyanga Opposite page: Chlorophyll concentration in the WIO during the austral summer (Dec 2011-Mar 2012). © Nasa/Suomi NPP/Norman Kuring INTRODUCTioN ference between gross primary production and respira- tion gives net primary production. Respiration is the The plankton release of carbon dioxide by photosynthetic organisms; Primary productivity may be defined as the amount of leaving a net photosynthetic fixation of inorganic carbon organic material produced per unit area per unit time; or into autotrophic biomass. Phytoplankton in the ocean simply as the product of phytoplankton biomass times contributes to roughly half of the planetary net primary phytoplankton growth rate (Cloern and others, 2014). production (Field and others, 1998). Through sinking of Marine primary production plays an important role in the fixed organic matter, primary production acts as a bio- food web dynamics, in biogeochemical cycles and in logical pump that removes carbon from the surface ocean, marine fisheries (Chassot and others, 2010, Passow and thereby playing a global role in climate change (ASCLME/ Carlson 2012). The term plankton, which means wander- SWIOFP 2012). ing or drifting, generally encompasses three groups: the The main types of phytoplankton are cyanobacteria, phytoplankton, the photosynthetic (plant) component; diatoms, dinoflagellates, green algae and coccolitho- the zooplankton, which is the animal component and the phores. In addition to phytoplankton, other primary pro- ichthyoplankton which comprise the fish larvae. How- ducers contribute to ocean primary production, especially ever, this chapter will concentrate on the plant compo- in the coastal areas. These include mangroves, seagrasses, nent, the phytoplankton. Phytoplankton is the foundation macroalgae and salt marshes (Oliveira and others, 2005, of the aquatic food web, meaning that they are the pri- Duarte and others, 2005). Furthermore, symbiotic algae, mary producers (Vargas and others, 2006). A common fea- some epiphytes and benthic microalgae are also produc- ture to all phytoplankton is that they contain chlorophyll-a; ers. However, phytoplankton contributes to more than 90 but there are other accessory pigments such as chloro- per cent of total marine primary production (Duarte and phyll-b and chlorophyll-c, as well as photosynthetic carot- Cebrian 1996). In the group of cyanobacteria, some gen- enoids (Kirk 1994, Barlow and others, 2008). These era such as Trichodesmium, Nostoc and Richelia, are able to pigments absorb solar energy and convert carbon dioxide fix nitrogen from the atmosphere, thereby increasing and water into high-energy organic carbon compounds sources of nutrients (Lyimo and Hamis 2008, Poulton and that fuels growth by synthesizing vital required compo- others, 2009). Under increased nutrient concentration, nents such as amino acids, lipids, protein, polysaccha- other genera from among the dinoflagellages such as rides, pigments and nucleic acids. The photosynthetic Dinophysis can form blooms that could be harmful (harm- process produces gross primary production; and the dif- ful algal blooms – HABs or red tides) or be a nuisance to Western Indian Ocean 207 FinalRESOCR.indd 207 15/09/16 13:08 IV . Assessment of major ecosystem services from the marine environment other aquatic organisms like fish and shellfish, the envi- The Western Indian Ocean (WIO) region ronment, as well as humans, and could cause serious eco- The WIO region could be sub-divided into two large nomic losses in aquaculture, fisheries and tourism-based marine ecosystems, namely: the Somali Current Large activities (Babin and others, 2008). Marine Ecosystem (SCLME) and the Agulhas Current A general classification of microalgae is illustrated in Large Marine Ecosystem (ACLME); the region also cov- Table 16.1. The table gives a total of 13 divisions/classes. ers areas that are outside the two LMEs (ASCLME/SWI- Size-wise, phytoplankton can be sub-divided into three OFP 2012). The ACLME is located in the southwestern groups, namely: pico-phytoplankton (0.2 – 2.0 µm), Indian Ocean, encompassing the continental shelves and nano-phytoplankton (2.0 – 20 µm) and micro-phytoplank- coastal waters of mainland states of Mozambique and ton (20 – 200 µm), see Table 16.1. The primary consum- eastern South Africa, as well as the archipelagos of the ers, zooplankton, also play an important role in the transfer Comoros, the Seychelles, Mauritius and La Reunion- of energy from one trophic level to another in the marine France (Heileman and others, 2008). At the northeastern food web, thereby acting as a link between producers and end of the ACLME is Madagascar, the world’s fourth larg- higher consumers. Zooplankton is sub-divided into two est island with extensive coastline of more than 5 000 km groups, namely, the protozoa zooplankton with two phyla (McKenna and Allen 2005). The SCLME extends from and Eumetozoan zooplankton with about 14 phyla. In the Comoro Island and the northern tip of Madagascar in zooplankton, the size ranges from 0.01 mm to several cen- the south to the Horn of Africa in the north, bordered by timeters, although most of them do not exceed 5 mm Somalia, Kenya and Tanzania. (Conway and others, 2003). Herbivorous zooplankton In the WIO region, circulation pattern differs between feed on phytoplankton, and these feed carnivorous zoo- the southern and the northern parts of the region. In the plankton, which in turn are eaten by other larger animals south, circulation is persistent throughout the year, being (see schematic illustration in Figure 16.1), thus playing a wind driven anti-cyclonically, whereas in the north circu- very important role in supporting the productivity of lation is forced by the monsoon winds that reverse sea- marine/coastal fisheries. Zooplankton is the basic food for sonally (Mengesha and others, 1999, Lutjeharms 2006). most fish larvae and some adult fishes as well as cetaceans. According to Lutjeharms (2006), the flow during the As such, its distribution is considered as a proxy for ocean north-east monsoon is directed southwards along the East fertility, providing information on potential fishing zones African coast and during the southwest monsoon it is (Reid and others, 2000). directed northwards, becoming the East African Coastal Table 16.1. Classification of phytoplankton algal types (adopted from Jeffrey and Vesk 1997) Microplankton Nanoplankton Picoplankton Algal Division/Class 20 – 200+ µm 2 – 20 µm 0.2 – 2 µm Bacillariophyta + + + Chlorophyta + + + Chrysophyceae + + + Cryptophyta + + - Cyanophyta + + + Dinophyta + - - Euglenophyta - + - Eustigmatophyta + + - Prasinophyta + + + Prochlorophyta - + - Prymnesiophyceae + - - Rhodophyta - + - 208 Regional State of the Coast Report FinalRESOCR.indd 208 15/09/16 13:08 16. Phytoplankton primary production Sunlight Other Predators Marine Mammals, Humans and Sharks Plant Plankton Bacteria Large Surface Feeding (Phytoplankton) (Pelagic) Fish Animal Plankton Larger Invertebrates (Zooplankton) and Small Fish Dead Matter Minerals Seabed Life Bottom Feeding (Demersal) Fish Figure 16.1. Simplified illustration of the marine food chain. In the real sense, the marine food web is very complex with interactions between organisms in the ecosystem, composed of detritus, numerous organisms such as bacteria, phytoplankton, zooplankton, fish, birds, marine mammals and humans. (Source: adapted from http://www.goldridge08.com). Current. This current originates from the bifurcation of and others, 1999). The phytoplankton communities are the East Madagascar Current, at the African coast around usually dominated by pico-phytoplankton and nano-phy- 100S. toplankton (Barlow and others, 1999, Barlow and others, Both LMEs are moderately productive with an aver- 2007, Goericke and others, 2000, Sá and others, 2013). age production ranging between 150 and 300 g C m-2 a-1 (Heileman and Scott 2008, Heileman and others, 2008). Factors affecting productivity and the distribution For the ACLME, primary production is largely driven by of primary producers the Mozambique Channel eddies and localized eddies or Primary production, phytoplankton distribution and abun- topography-driven upwelling (Quartly and others, 2005, dance are influenced by several factors, thus they vary both Kyewalyanga and others, 2007, Barlow and others, 2013). seasonally and spatially. Primarily, phytoplankton depends Relative to other parts of the Indian Ocean and the Atlan- on carbon dioxide, sunlight and nutrients for growth, but tic Ocean that borders western Africa that seem to be some other factors such as water depth, water temperature, more productive (Figure 16.2), productivity in the WIO wind and grazers also play a significant role. The South and region is between low and moderate levels, although Western Indian Ocean are probably the least studied areas. there are some highly productive areas, especially along Research has so far focused on the Arabian Sea (northwest- the coasts. In summary, the WIO waters are characterized ern Indian Ocean) while extensive regions of oceanic by warm temperatures and low nutrients, low biomass waters remain unknown. Therefore, in the WIO region, and relatively low-moderate primary production (Karl information on primary production and phytoplankton Western Indian Ocean 209 FinalRESOCR.indd 209 15/09/16 13:08 IV . Assessment of major ecosystem services from the marine environment 40ºW 20ºW 0º 20ºE 40ºE 60ºE 80ºE 50ºN 30ºN 10ºN 10ºS 30ºS SeaWIFS Chlorophyll (mg/m3)