Phenology is a key dimension of plankton community dynamics, yet the seasonal organization of microzooplankton communities remains poorly resolved because most monitoring programs rely on monthly or biweekly sampling. This limitation is particularly important for planktonic ciliates, which have short generation times and may respond rapidly to environmental variability. Here, we examined community-level phenology of tintinnids based on a high-frequency year-round survey conducted three times per week from February 2024 to February 2025 in Qingdao coastal waters, Yellow Sea. A total of 149 samples were collected, and 30 tintinnid species were identified.
Using normalized temporal abundance profiles and cluster analysis, 24 common species were classified into four phenological types: cold-water type, temperate type I, temperate type II, and warm-water type. These types exhibited distinct seasonal occurrence windows, abundance peaks, and thermal performance ranges. Community-level clustering further resolved four recurring seasonal assemblages, each dominated by one phenological type. Thus, the annual succession of the tintinnid community was structured as an ordered replacement of phenological types with different thermal niches, rather than a simple fluctuation in total abundance.
Thermal performance curve analysis showed that the effective breadths of thermal performance ranges were broadly comparable among phenological types, generally within 5–8°C. By reanalyzing published tintinnid datasets from Ningbo and Hong Kong using the same framework, we found that the number of phenological types increased with the magnitude of local annual seawater temperature range: two types in Hong Kong, three in Ningbo, and four in Qingdao. Cold-associated types showed positively skewed thermal performance curves, indicating higher sensitivity to low-temperature fluctuations, whereas warm-associated types followed the more typical negatively skewed pattern.
Our results demonstrate that high-frequency observations are essential for resolving microzooplankton phenology and short seasonal turnover. By linking local thermal regimes, thermal niches, and phenological types, this study provides a transferable framework for comparing community-level plankton phenology across coastal systems under changing ocean conditions.
 

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