The Yellow Sea, as one of the four marginal seas in China, is an area where land, ocean and atmosphere interact more intensely with concentrated human activities and marine economic development. It owns abundant natural resources and developed coastal economy. Previous studies found that the Yellow Sea was intermittently restricted by phosphorus (P) [1
], which leads to a constant changing process of protists trophic structure in this ocean area, and then altered the ecological functions of protists, such as nutrients utilization and transformation [3
]. Sand-dust deposition is an important pathway for transporting land-based nutrients and pollutants to the ocean and providing nutrients for the marine planktonic protists. Researchers found that sand-dust deposition held a significant positive correlation with chlorophyll a
and primary productivity [5
]. Asian dust is an important part of global dust, and the Yellow Sea, located in the downwind zone of the Asian dust source area, is the greatest probability of being affected by Asian dust in China’s offshore waters. Some studies showed that Asian dust deposited into the Yellow Sea [6
] through a long-distance transportation in sand-dust weather, and obvious “fertilization” phenomenon was observed to affect the primary productivity. Thus, Asian sand-dust deposition is an important factor that affects the primary productivity of the Yellow Sea. The nutrients carried by sand-dust [9
] could alleviate the phosphorus deficiency in the Yellow Sea, thereby affected the abundance and trophic structure of marine biota. However, the effect of sand-dust on the growth of nanoplanktonic protists in the P-limited Yellow Sea has not been reported.
Marine nanoplankton with size ranging 2–20 µm, is a key component of marine micro-food web and they play an irreplaceable role in maintaining primary productivity and material cycle [13
]. It is remarkable that different size groups of nanoplankton (e.g., 2–4, 4–5, 5–7, 7–10 and >10 µm) revealed different roles in the food web due to the variations of species and proportion [16
]. As such, three size groups, i.e., 2–5, 5–10, and 10–20 µm of nanoplankton were proposed in this study. According to the mechanisms of energy and nutrient acquisition, nanoplanktonic protists are divided into autotrophic, heterotrophic (i.e., protozoa) and mixotrophic protists [2
]. Autotrophic nanoplanktonic protists are the key contributor to marine primary productivity [17
], Heterotrophic nanoplanktonic protists affect the community structure and function of nanoplankton by preying on bacteria, cyanobacteria or smaller protists [18
], and being preyed by medium-sized zooplankton [20
], and then flow to a higher trophic level from the bottom of the food web [22
]. Mixotrophic nanoplanktonic protists own both the autotrophic and heterotrophic mode, and improve the utilization of nutrients by changing nutritional habits [2
]. Hence, the material conversion and energy flow of the marine plankton ecosystem depends on different trophic marine nanoplankton protists. Therefore, it is of great significance to investigate the effects of dust and phosphorus input on the abundance and trophic structure of planktonic protists.
However, there are several blind spots that need to be further revealed in the relationship between the sand-dust and planktonic protists. Firstly, the effect of sand-dust input on nanoplanktonic protists is unknown. Though the sand-dust can provide nutrients during the transport process, it also adsorbs substantial heavy metals such as copper, cadmium, plumbum, and other land-based pollutants, which present a strong toxic effect on plankton [24
]. Secondly, whether the sand-dust, as a supplement of nutrients, can effectively compensate for P limitation in the Yellow Sea is still unclear. Thirdly, it is difficult to accurately distinguish the ecological functions of different species due to the complexity of marine nanoplanktonic protists.
Hence, in this study, to make a better understanding of the effects of sand-dust and phosphorus on the abundance and trophic structure of different sizes of planktonic protists, an experiment was conducted onboard in the southern Yellow Sea where P was deficient., the corresponding changes of nanoplanktonic protists were following investigated according to the size groups (i.e., 2–5, 5–10, and 10–20 µm) and trophic types (i.e., autotrophic, heterotrophic and mixotrophic protists) [3
]. The results could provide a scientific supplement for revealing the effects of sand-dust deposition on the trophic structure and ecological function of marine planktonic protists. Meanwhile, this study supply a supplement for further analyzing the effects of sand-dust deposition on marine ecosystems.
According to the changes of relative abundance and trophic structure of protists by the rank sum test of Kruskal-wallis and Nemenyi-Wilcoxon-Wilcox, it showed that P promoted the growth of protists with the different particle sizes and the trophic modes at the early stage of culture. While the sand-dust presented different effects on the growth of different trophic modes. It stimulated the autotrophic but inhibited the heterotrophic and mixotrophic protists. The order of relative abundance of autotrophic protists with different particle sizes was 10–20 µm > 5–10 µm > 2–5 µm. The order of decreasing the proportion of heterotrophic protists with different particle sizes was 5–10 µm > 10–20 µm > 2–5 µm; and the order of inhibiting the proportion for mixotrophic protists with different particle sizes was 10–20 µm > 5 µm > 2–5 µm. The early stage of sand-dust deposition obviously inhibited the heterotrophic and mixotrophic protists by the dissolution of toxic and harmful substances in sand-dust. Mixotrophic and heterotrophic protists are the primary predator of autotrophic protists; Pearce et al. (2011) found 42%~82% primary production was consumed by mixotrophic and heterotrophic protists [35
]. The decrease of mixotrophic and heterotrophic protists could accelerate the growth of autotrophic protists. In this study, sand-dust addition promoted the growth of autotrophic protists at the early stage, and the decrease of mixotrophic and heterotrophic protists accelerated this process. The further study confirmed that sand-dust storm could significantly induce the occurrence of red tide in the southern Yellow Sea [18
]. Besides, the harmful inhibiting effect of sand-dust on heterotrophic protists community would weaken the marine matter cycle and food transfer efficiency.
At the late stage of culture, P presented a prohibitive effect on the growth of protists, which was related to the rapid growth of protists at the earlier stage, which resulted in nutritional deficiency and growth restriction at the late stage. In the late stage of sand-dust deposition, the autotrophic protists were limited due to the excessive growth of autotrophic protists in the earlier stage. At the late stage, sand-dust demonstrated stimulating effect for heterotrophic and mixotrophic protists, and the order was heterotrophic > mixotrophic protists. As for the heterotrophic protists with different particle sizes, the order was 5–10 µm > 2–5 µm > 10–20 µm, and for mixotrophic protists, the order was 10–20 µm > 5–10 µm > 2–5 µm. In the late stage of sand-dust deposition, the toxicity of harmful substances to heterotrophic protists gradually weakened. These protists slowly adapted to the sand-dust environment and dissolution of organic substances and other nutrients in sand-dust provided abundant nutrients for the heterotrophic protists [17
]. As such, their abundance maintained stable and their proportion gradually increased. The effects of low sand-dust group and high sand-dust group on the proportion of heterotrophic and mixotrophic protists were basically coincident. However, the effect of high sand-dust group was stronger and the nutritional supplementation lasted longer in the later period.
For the trophic structure, at the early stage of incubation, P addition could promote all kinds of phytoplankton protists, especially the 10–20 µm autotrophic protists, which were consistent with that of autotrophic protists at the early stage of sand-dust deposition. At the late stage of incubation, both P addition and sand-dust deposition restricted all sizes of autotrophic protists. Therefore, it is speculated that the effect of sand-dust deposition on the autotrophic protists in this ocean were ascribed to the dissolution of P from sand-dust. The early promotion of sand-dust was to supplement the P in the sea area, while at the late stage, the inhibition for the autotrophic protists was mainly related to the early rapid growth of autotrophic protists by consuming up the P and other nutrients. The limiting capacity of the sand-dust group at the late stage is less than that at P-adding groups. This reflected that the dust could continuously supply the P and other nutrients.
The abundant P at the early stage of sand-dust deposition promoted the rapid growth of large-size autotrophic protists. This phenomenon will further accelerate the occurrence of red tides in eutrophic sea areas. Sand-dust deposition in the late stage stimulated the small-size heterotrophic protists and accelerated the material cycle efficiency and food transfer capacity in the sea. Therefore, the influence of sand-dust on the structure of different particle size and trophic protists will change the structure of the micro-food web in the ocean. It will also change ability of material transformation in the water body and ultimately, and affect the ecological function of protists in the transforming matter and producing food in the sea.
(1) Sand-dust deposition affected the trophic structure of different particle sizes of planktonic protists in the southern Yellow Sea. This could lead to change the structure of micro-food webs in the sea, and affect the ecological functions of micro-food webs in material transformation and food production.
(2) The growth of planktonic protists of all trophic modes in this ocean was restricted by P. The early addition of P could promote the growth of planktonic protists of all trophic modes in the southern Yellow Sea. The effect on the10–20 µm autotrophic protists was most obviously, while the late addition of P mainly restricted the growth of different sizes of protists.
(3) The effect of initial sand-dust deposition on autotrophic protists was the same as that of P, it inhibited heterotrophic and mixotrophic protists. The positive effect of sand-dust deposition on heterotrophic and mixotrophic protists was strong at the late stage, and it improved the abundance of small-sized heterotrophic protists.
(4) The positive effect of sand-dust deposition on autotrophic protists in the Yellow Sea might be related to the dissolution of P from the sand-dust. The promotion of small-size heterotrophic protists in the late stage of sand-dust deposition could accelerate the material circulation efficiency and food transformation in the sea.