UNDERSTANDING ECOSYSTEM PROCESSES IN THE BERING SEA 2007–2013

The Spring Bloom Matters THE IMPORTANCE OF THE SPRING BLOOM TO THE OVERALL PRODUCTIVITY OF THE BERING SEA The success of the highly produc- the spring bloom to the Bering Sea What We Found tive Bering Sea fishery depends on ecosystem and to predict how these We found the spring ice-associ- massive blooms of tiny, single-celled blooms might be altered for better ated bloom (Figure 1) to be of vital plants that occur each spring when or worse in a warmer Bering Sea. importance to the productivity of increasing light and abundant nutri- the Bering Sea. It begins the plank- ents enable these plants to flourish What We Did ton growing-season and supplies a both in the ice (ice algae) and in the We collected samples over a large large and dependable food source water column (phytoplankton) as region of the shelf, using ice cores, to which the life cycles of many of the sea ice retreats. These blooms water samplers and nets to identify the important zooplankton, and support a zooplankton commu- and quantify the biomass of the continued on page 2 nity that has just awoken from a planktonic ecosystem components. Fig. 1 period of rest during the long, dark, Shipboard experiments measured cold winter. This community is important biological rates, such as made up of the small, unicellular zooplankton feeding, growth and microzooplankton and the larger, reproductive rates. Datasets then multicellular, mostly crustacean were integrated and synthesized to mesozooplankton dominated by determine regional patterns and year- copepods and krill. The zooplank- to-year variability in the biomass, pro- ton community, in response to the ductivity and consumption rates of spring bloom, dramatically increases different planktonic components. A its numbers and biomass and pro- new planktonic ecosystem model was vides an abundant, highly nutritious developed to better understand the food source for seabirds, mammals food web dynamics and to predict the and fish. This project seeks to bet- response of the planktonic ecosystem ter understand the importance of to future climate changes.

The Bering Sea planktonic food web in spring (not to scale). (Top) A mixed diatom assemblage The Big Picture from the Bering Sea spring bloom. Diatoms are Using a modeling approach combined with extensive data collection and at-sea experiments, the dominant component of both ice algal and we tackled a conundrum: why do years with warm ocean temperatures result in low overall success phytoplankton communities during spring. Photo of copepods and other large zooplankton, despite better food supplies and faster growth? Copepod credit: E. Sherr. (Middle) Dinoflagellates like this overall success is extremely sensitive to winter prey concentrations, even though those concentrations Protoperidinium sp (left) and ciliates similar to the Leegardiella sp. (right) are important members of are orders of magnitude lower than those during the spring bloom. Increased mortality during warm the microzooplankton communities. Photo credit: years and/or lower growth rates at warm temperatures that exceed the optimal thermal tolerance for E. Sherr. (Bottom) The copepod Calanus glacialis/ the animals also are important. Our model provides almost as many new questions as answers, and marshallae (left) and the euphausiid (krill) Thysa- can be used to guide future research directions noessa raschii (right) are dominant components of the mesozooplankton. Photo credit: C. Gelfman.

LINKING SEA-ICE RETREAT TO PLANKTON COMMUNITY STRUCTURE AND FUNCTION IN THE BERING SEA: DATA SYNTHESIS, BIOPHYSICAL MODELING, AND MULTI-DECADAL PROJECTION A component of the BEST-BSIERP Bering Sea Project, funded by the National Science Foundation and the North Pacific Research Board with in-kind support from participants. benthic, species are timed. result in tighter coupling between Fig. 2 The planktonic food web in the the planktonic producers and Bering Sea is far more complex consumers with detrimental conse- than simply large zooplankton quences for the benthic (sea floor) feeding on large phytoplankton. community. Copepods and krill all readily feed Our planktonic ecosystem on ice algae, phytoplankton and model indicates that large zoo- microzooplankton. Frequently plankton have greater success in microzooplankton, not phyto- cold years in spite of, not because plankton, are their preferred food. of, spring-summer conditions The life cycles of many impor- (Figure 2). In warm years, total tant zooplankton species are primary and microzooplank- timed to take advantage of the ton production are higher and spring bloom. Peak reproduc- warmer temperatures mean that tion of the large copepod Calanus growth and development of the glacialis/marshallae coincides with large zooplankton are faster. Yet the bloom. Adult females that the large zooplankton have lower have survived the food-limited overall success in warm years. A winter are mature and ready to new model of copepod life history take advantage of the rich bloom tradeoffs is narrowing down the food environment. These animals potential reasons for this. One idea respond rapidly to the increased is that warm winter temperatures food supply by producing up to 50 decrease copepods’ overwinter- eggs per female per day. The eggs ing success by making them burn and early developmental stages of through their energy reserves too copepods are an important food fast, but the model suggests that source for larval fish. this direct temperature effect is The spring bloom provides an outweighed by the positive effect almost inexhaustible food supply of higher temperatures on spring- allowing copepods to increase their summer growth and development. biomass by up to 10-fold between Another idea is that cold years early spring and summer. Even so, might favor the production of ice the zooplankton community leaves algae prior to the spring bloom. much of the spring bloom produc- Robert Campbell, University of Rhode Island tion un-grazed. This excess produc- Carin Ashjian, Woods Hole Oceanographic Institution tivity falls to the sea floor where it Neil Banas, University of Washington supports a rich benthic community Evelyn Lessard, University of Washington including commercially impor- Alexei Pinchuk, University of Alaska Fairbanks tant crustaceans such as king crab. Barry Sherr, Oregon State University Evelyn Sherr, Oregon State University What if a future warmer ocean Jinlun Zhang, University of Washington upsets this balance? We believe Model results. Time evolution of an intense ice-edge that one reason that the spring bloom, from spring-summer 2009 observations bloom is so productive is that it The Bering Sea Project is a partnership between (red) and the model (gray). Gray lines show the North Pacific Research Board’s Bering Sea gets a jump on the zooplankton modeled community evolution along Lagrangian Integrated Ecosystem Research Project and the transport pathways that intersect the observed late grazers, which are not very abun- National Science Foundation’s Bering Ecosystem April bloom; the spread among them shows the dant after the long winter and Study. www.nprb.org/beringseaproject effect of small-scale patchiness and variability in cannot consume all of the primary ice-retreat timing. Observations are courtesy of the production. A warmer ocean could Mordy, Lomas, Sambrotto, and Sherr groups.

LINKING SEA-ICE RETREAT TO PLANKTON COMMUNITY STRUCTURE AND FUNCTION IN THE BERING SEA: DATA SYNTHESIS, BIOPHYSICAL MODELING, AND MULTI-DECADAL PROJECTION A component of the BEST-BSIERP Bering Sea Project, funded by the National Science Foundation and the North Pacific Research Board with in-kind support from participants.