Light response and acclimation of microalgae in a changing Arctic

Abstract

Summary of thesis:

This thesis investigates ice-associated and under-ice algal communities, and the controlling role of light in the Arctic pack ice region. Changes in the light regime are a consequence of the substantial environmental changes happening in the Arctic. To this end the research vessel Lance was frozen into the pack ice north of Svalbard (80.2–82.8 °N) for nearly half a year from January to June 2015 during the Norwegian young sea ICE (N-ICE2015) expedition to study the new, thinner sea ice regime and associated environmental and ecosystem processes. The presented results from the spring and early summer season (April–June) suggest that an under-ice phytoplankton bloom can develop and grow under the opaque ice cover because open and refrozen leads acted as light conduits into the water column. The ratio of photoprotective (PPC) to photosynthetic (PSC) carotenoids indicated that the bloom was low-light acclimated, which supports the conclusion of in situ growth under the ice pack. The pigment ratios were in addition related to the slope of in situ absorption measurements between 488 and 532 nm (affected by absorption by PPC and PSC, respectively) to evaluate a method to assess the photoacclimation state of phytoplankton blooms in situ. We also studied the young ice in a refrozen lead which had a thin ice and snow cover and high light transmittance of up to 0.41. Under-ice irradiance, Ed(PAR) (photosynthetically active radiation, 400 to 700 nm) was up to 350 µmol photons m-2 s-1, and the high light conditions resulted in accumulation of cellular photoprotective pigments (mycosporine-like amino acids (MAAs) and PPC). Biomass in the lead ice (mg Chl a m-2) did not exceed the surrounding older and thicker ice with low light availability (Ed(PAR) <20 µmol photons m-2 s-1), owing likely to several factors such as time and energy needed for photoacclimation, nutrient limitation and time needed for recruitment of ice algae into the new ice habitat (the refrozen lead). The algal community in the young ice was in the beginning composed of ciliates, flagellates and dinoflagellates, and over one month developed towards dominance of typical ice-associated pennate diatoms (e.g. Nitzschia frigida and Navicula spp.). Environmental conditions such as irradiance levels did not affect the species succession to a great degree, based on multivariate statistical testing with the main environmental drivers and comparison to the surrounding older and thicker ice. This suggests that species traits and adaptations to the ice environment play an important role for algal community dynamics in sea ice. These studies improve our understanding on ice algal ecology in a high-light habitat and offer new insights into community development in newly formed sea ice in spring. The origin of the sea ice diatoms in the young lead ice was in the surrounding older ice. We also discuss the observed patterns – algal blooms in the different habitats – with respect to the physical changes in the icescape. Thinning and increased drifting speed of the sea ice affects the dynamics in the icescape and may lead to more frequent lead formation, increasing the importance of these habitats in the future Arctic.

Sammendrag:

Hvordan responderer mikroalger på økt lystilgjengelighet i Arktis?

Havisområdene i Arktis er i stor endring, som vises blant annet ved at isdekket minker, blir tynnere og yngre. Alt dette påvirker levestedet til isalger og planteplankton, dvs. mikroskopiske planter som lever i den porøse havisen og i vannsøylen under, og som danner basen i næringsnettene. For eksempel blir det mer lys tilgjengelig når isdekket minker siden is, og særlig snø, effektivt blokkerer sollysets gjennomtrengning. Foreløpig finnes det relativt lite kunnskap og forskning på dette fra Arktis, spesielt utenfor sommermånedene. For å finne ut hvordan algene responderer på disse endringene, ble forskningsfartøyet Lance frosset inn i polisen i et halvt år for å studere atmosfæren, snø- og isdekket, havet under isen, og økosystemene. Vi tok prøver fra både vann og is, og analyserte blant annet artssammensetning, næringssalter og algepigmenter. Vi målte også lysnivåene i de forskjellige levestedene og fysikalske variabler som salinitet og temperatur. Vi fant ut at planteplankton kunne vokse under den tykke isen med mye snø, og følgelig lite lystransmisjon, hjulpet av råker som fungerte som vinduer i isen med høy lystransmisjon. Vi studerte også de optiske egenskapene i tynn, ung is i en råk i vårt studieområde, og fant ut at lystransmisjonen var betydelig høyere enn i den omliggende tykke, gamle isen (5-40 % sammenlignet med <0.003 %). Isen i råka fungerte også som levested for isalger. Algesamfunnet utviklet seg til et typisk isalge-samfunn med pennate kiselalger som dominerende arter, til tross for mye høyere lysnivåer enn i den til nå vanligere tykke isen. Biomassen var ikke høyere, som tyder på at dette levestedet ikke hadde økt primærproduksjon til tross for høyere lystilgjengelighet. Våre funn gir ny informasjon om isalgene i et lite studert område og årstid, og antyder at tap av flerårsis kan gi redusert spredning og vekst av isalger.