Study the Science: Plankton
The planktonic communities program describes spatial, seasonal and interannual characteristics of the phytoplankton, zooplankton, and microplankton communities within the study areas.
Phytoplankton, along with algae that grow attached to sea ice, are the conduit of the sun´s energy into organic "food” at the base of the food chain in the Chukchi Sea. Phytoplankton are assessed as chlorophyll-a concentration from water samples collected with a CTD (conductivity-temperature-depth) rosette. We also measure the concentration of the major nutrients required for their growth in these samples. Zooplankton are the major consumers of phytoplankton, and hence the key pathway to vertebrates such as pelagic fish, seabirds, and even bowhead whales. Zooplankton are collected using two different methods: paired ring nets of medium-size mesh hauled vertically at slow speed for smaller species, and Bongo nets towed off the side of a ship moving at 2 knots for larger, faster species.
There are more multi-cellular zooplankton per square kilometer in the Chukchi Sea than there are stars in the Milky Way (200-400 Billion).
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The expanded study area of 2011 was repeated again in 2012, and provided a complete contrast to the previous year. The cold ocean conditions and delayed ice-retreat in 2012 resulted in high nutrient concentrations, with phytoplankton biomass and composition indicating a seasonal phytoplankton (algal) bloom. In 2012 we added observations on microzooplankton (single-celled animals) and determine that they were as important a food resource to the multi-cellular zooplankton as were the phytoplankton themselves. These conditions supported the largest biomass (weight) of copepods (a tiny shrimp-like animal) observed in the past five years. These copepods also consisted of generally larger cold-loving species than observed in previous sampling years. The distribution of zooplankton species and their community composition was highly related to the distribution of water properties, specifically temperature and to a lesser degree salinity. Combined with observations from other years, it becoming clear that differences in the timing of ice-melt, water temperatures, transport of water masses, nutrients and chlorophyll (algal biomass) are coupled to the large inter-annual differences observed in the planktonic communities over the past 5 years.
The study area was greatly expanded in 2011, allowing us a unique look at the distribution of zooplankton in the northeastern Chukchi Sea. Water temperatures suggest the region was flooded with Bering Sea water in August resulting in warmer waters that favored larger open ocean Pacific species. Chlorophyll and nutrient were low in surface waters, with deeper nutrient pools observed in August that became exhausted by September. The zooplankton community generally consisted of the same species observed in previous years, with abundance and biomass of copepods higher than average in August, but lower in September. Despite warmer water temperatures, an interesting exception to previous years occurred in 2011 with the transport of the Arctic basin copepod species Calanus hyperboreus into the study area during a period of sustained upwelling in Barrow Canyon.
Water temperatures were intermediate between the previous years, as were chlorophyll and nutrient concentrations. Similar to prior years a total of about 80 categories of zooplankton, were observed during the 2010 field year. Both holozooplankton (full-time zooplankton) and particularly meroplankton (larval stages of benthic organisms) abundance and biomass were much higher in 2010 than in previous years, especially for larger-bodied animals. The addition of a third study area helped broaden our perspective on pattern in the northeastern Chukchi Sea. We are starting to believe differences in ice-melt timing, water temperatures, northward transport of water masses, nutrients and chlorophyll are influencing the large inter-annual differences observed in the planktonic communities over the past 3 years.
Water temperatures were warm and sampling began well after the spring phytoplankton bloom. In total, 70 taxonomic categories of zooplankton, including 11 meroplanktonic larval categories, were observed during the 2009 field year, and consistently mostly of species observed the previous year. The abundance and biomass of holozooplankton has higher than in 2008, while that of meroplankton was lower than in 2008. As observed in 2008, differences in zooplankton communities could be distinguished between the study sites, and a seasonal evolution of the community structure was apparent over both areas.
Due to the late ice retreat, the phytoplankton spring bloom was delayed. In total, 76 taxonomic categories of zooplankton were observed, including an unexpected and substantial contribution of meroplankton. The greatest taxonomic diversity was observed within the copepods (20 species), followed by the jellyfish (9 species), with species typical for the region and largely of Pacific origin. Abundance and biomass estimates of the zooplankton community appeared lower than typical for the region, perhaps due to relatively cold oceanographic conditions experienced during 2008, which slowed the normal growth and development of the zooplankton. Despite the relative proximity of the study areas, they could generally be separated based on community structure. Not surprisingly, a seasonal evolution of the community structure was apparent over both study areas.