Reproductive rates of small copepods in a changing Arctic: present and future?

Abstract

Reproductive rates of zooplankton are essential to calculate secondary production of any ecosystem. Temperature is recognized as a major factor influencing reproduction in copepods. Projected warming of Arctic surface waters is estimated between 1.8 and 4°C by 2100. Its suggested consequence is a shift from dominance of larger Arctic copepod species to smaller temperate species in Arctic food webs. The temperature-responses of reproductive rates is well-documented for larger copepods but is understudied for many small copepods species (body length < 1 mm). We investigate the reproduction at cold temperatures of three small copepod species with different reproductive strategies. Egg-hatching experiments were conducted on the egg-carrying Oithona similis and Microsetella norvegica, and egg production experiments were conducted on the broadcast-spawner Microcalanus spp. To the best of our knowledge, our study is the first to present egg-hatching rates for M. norvegica and egg production rates of Microcalanus spp. at low temperatures. Specimens were collected in a high-latitude fjord and incubated at temperatures between 1.3 and 13.2°C in June and August 2017 and March, May, June and August 2018. Typically, an egg-carrying copepod shows a linear increase between temperature and hatching rate. Accordingly, the egg-hatching rates of O. similis increased from 5% day-1 at 1.3°C to 34%.day-1 at 13.2°C. However, the egg-hatching rates of M. norvegica showed a bell-shaped relationship within the temperature range with a peak at 7°C. The average daily egg-production of the broadcast-spawner Microcalanus was 9  3 in May 2018, 6  5 in June 2018 and 12  8 in August 2018 and was temperature independent. The egg hatching time of Microcalanus spp. decreased with increasing temperatures, ranging from 4.3 days at 3°C to 1.6 days at 9.8°C. These results indicate that temperature affects small copepods species differently. While O. similis’ egg-hatching is strongly affected by temperature, additional factors, such as seasons, may influence M. norvegica reproduction. Microcalanus spp. egg production responded to seasonality while its hatching time responded to temperature. Therefore, the predicted increase in Arctic surface waters temperatures will positively affect the reproduction of small copepods irrespective of their reproductive strategies. Because larger Arctic copepod species are negatively affected by an increase in temperature, our study supports the idea of a shift towards smaller copepods species in the Arctic pelagic ecosystem.