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

气候变暖预计将使波罗的海的水面温度上升2-3摄氏度，2016年已经比波罗的海本土(阿科纳和哥德兰海)的长期平均水平(HELCOM 2017)高出2摄氏度。自1980年代末以来，观察到波罗的海的硅藻现存量急剧减少，自养甲藻的优势地位发生了变化。根据Wasmund等人的说法。(2017)，唯一的原因应该是气候变暖，即较高的水温。据推测，由于冬季气温变暖，中游动物(桡足类、轮虫)和微型浮游动物(纤毛虫、异养甲藻)的早期发育导致春季水华早期阶段的放牧加剧，这些生物以小硅藻为代表，例如海滨骨骼藻，这解释了过去几十年中硅藻数量减少的原因。然而，所描述的相互关系仍然鲜为人知，对波罗的海本身来说也完全不清楚。值得注意的是，波罗的海本身没有对微型浮游动物进行详细监测，2月、3月和5月对波罗的海本身进行的不频繁监测仍然在春季水华期间的浮游生物生物量知识方面留下了很大空白，所有这些都不能证实所提出的自上而下的设想。因此，我将把对整个浮游生物群落的高频监测与基于船舶的温度操纵实地实验和养殖研究相结合，以a)提供浮游生物群落动态的高分辨率数据，并b)评估水温升高、浮游动物群落变化和波罗的海本土春季水华期间硅藻生物量显著下降之间的机制联系。在项目巡航期间(2020年4月)的船上实验中，将通过在实验的一个部分包括一个仅由微型浮游动物组成的自然浮游动物群落，以及在环境和+3°C升温条件下的第二个设置中由微型和中浮游动物组成的群落(自然和双倍丰度)来研究放牧的影响。将进行亚稀释实验，以确定和选择微型浮游动物的捕食率。此外，将在监测(B部分)的帮助下，研究春季水华浮游生物的群落动态和物种组成。应该证明，在浮游植物春季繁殖期间，微型浮游动物和中游动物的数量是否已经足够多，能够控制硅藻的现存量。浮游植物、微型浮游动物和中游动物样本将于2020年4月上半月在我们自己的项目邮轮上采集，在IOW定期监测邮轮(2月、3月、5月)期间采集，并在同年4月下半月与GEOMAR邮轮合作采集。在实验室实验(C部分)中，将使用浮游生物培养在物种水平上更详细地分析气候变暖和关键食草动物的影响。

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.