Can micro- or mesozooplankton control phytoplankton spring blooms in the Baltic Proper under climate warming?

预计气候变暖将使波罗的海水面温度升高2-3℃，2016年波罗的海地区（阿科纳海和哥特兰海）的水面温度已比长期平均温度（HELCOM 2017）高出2℃。自20世纪80年代末以来，观察到波罗的海地区硅藻存量急剧减少，自养鞭毛藻占主导地位。根据Wasmund等人（2017）的说法，唯一的原因应该是气候变暖，即水温升高。据推测，由于冬季温度较温和，中浮游动物（桡足类、轮虫）和微型浮游动物（纤毛虫、异养鞭毛藻）的早期发育导致了对春季水华早期阶段的加强放牧，这是由马里诺骨藻等小硅藻代表的，并解释了其丰度在过去几十年中减少的原因。然而，对于波罗的海地区来说，所描述的相互关系仍然知之甚少，完全不清楚。值得注意的是，在波罗的海Proper没有对微型浮游动物进行详细的监测，并且在2月、3月和5月对波罗的海Proper进行的不频繁的监测巡航仍然给春季浮游生物生物量的知识留下了很大的空白，所有这些都不能证实所提出的自上而下的情景。因此，我将把整个浮游生物群落的高频监测与基于船舶的温度操纵现场实验和培养研究相结合，以a)提供浮游生物群落动态的高分辨率数据；b)评估水温升高、浮游动物群落变化与波罗的海Proper春季水华期间硅藻生物量显著减少之间的机制联系。在项目巡航期间（2020年4月）的船上实验中，将通过在实验的一部分中包括一个只有微型浮游动物的天然浮游动物群落，以及在环境和+3°C增强温度制度下的第二个设置中包括一个由微型和中型浮游动物（天然丰度加倍）组成的群落来研究放牧的影响。将进行次稀释实验，以确定和选择微型浮游动物的掠食率。在此基础上，通过监测（B部分）对春华浮游生物群落动态和物种组成进行研究。小型浮游动物和中浮游动物的数量是否已经足够丰富，足以控制浮游植物春季华时硅藻的存量，这一点应该得到证明。浮游植物、微型浮游动物和中浮游动物样本将于2020年4月上半月在我们自己的项目巡航中采集，在IOW定期监测巡航期间（2月、3月、5月），并与同年4月下半月的GEOMAR巡航合作。在实验室实验（C部分）中，将使用浮游生物培养物在物种水平上更详细地分析气候变暖和主要食草动物的影响。

Climate warming is predicted to increase the water surface temperature of the Baltic Sea by 2-3°C and was already up 2°C higher in 2016 compared to the long-term average (HELCOM 2017) in the Baltic Proper (Arkona and Gotland Sea). Since the late 1980s a dramatic decrease of the diatom standing stock and the change to a dominance of autotrophic dinoflagellates in the Baltic Proper have been observed. According to Wasmund et al. (2017), the only reason should be climate warming i.e. higher water temperatures. It is hypothesized that earlier development of the mesozooplankton (copepods, rotifers) and microzooplankton (ciliates, heterotrophic dinoflagellates) due to milder winter temperatures lead to intensified grazing on the early stages of the spring bloom, which are represented by small diatoms, e.g. Skeletonema marinoi, and explain its decreased abundances during the last decades. However, the described interrelations are still poorly understood and are totally unclear for the Baltic Proper. Notably, microzooplankton is not monitored in detail in the Baltic Proper and the infrequent monitoring cruises to the Baltic Proper in February, March and May still leave large gaps in the plankton biomass knowledge during spring bloom, all of which do not allow to confirm the proposed top-down scenarios. Thus, I will combine high-frequency monitoring of the whole plankton community with ship-based temperature-manipulated field experiments and culture studies to a) provide high resolution data of plankton community dynamics and b) assess the mechanistic link between increased water temperature, zooplankton community changes and the significant decrease in diatom biomass during the spring bloom in the Baltic Proper. In ship-based experiments during project cruise (April 2020), the effects of grazing will be researched by including a natural zooplankton community of only microzooplankton in one part of the experiment as well as by a community comprising micro- and mesozooplankton (natural and doubled abundance) in a second set-up under ambient and +3°C enhanced temperature regimes. A sub dilution experiment will be done to identify and select microzooplankton grazing rates. Further, with the help of monitoring (part B), the spring bloom plankton community dynamics and species composition will be researched. It should be proved if microzooplankton and mesozooplankton are already abundant in quantities able to control the diatom standing stock during phytoplankton spring bloom. Phytoplankton, microzooplankton and mesozooplankton samples will be taken at our own project cruise in the 1st half of April 2020, during IOW regular monitoring cruises (February, March, May) and in co-operation with a cruise of GEOMAR in the 2nd half of April of the same year. In laboratory experiments (part C), the effects of climate warming and key grazers will be analysed in more detail on species level using plankton cultures.