Identifying thresholds for ecological communities are of great importance for ecological application and management (
Townsend et al., 2008;
Martin and Kirkman, 2009). In this study, TITAN revealed that phytoplankton community in the coastal waters of northern Zhejiang Province, East China Sea substantially responded to TN, TP and pH gradients in a nonlinear way (
Fig. 3), suggesting the existence of complex stressor-response patterns. Both negative (z–) and positive (z+) thresholds were identified in the two seasons (
Table 3). Previous studies have also recorded the thresholds of phytoplankton community across multiple environmental gradients (
Smucker et al., 2013;
Cao et al., 2016;
Taylor et al., 2018).
Tang et al. (2016) suggested that thresholds of epilithic diatom assemblages in responses to TN and TP in the Three Gorges Reservoir were 0.382 mg/L (z–), 1.298 mg/L (z+) and
0.0160 mg/L (z–),
0.0650 mg/L (z+) in spring, respectively. Results of
Mazzei and Gaiser (2018) documented thresholds of diatom assemblages in response to TP gradient were
0.0487 mg/L (z–) and
0.2650 mg/L (z+),
0.0453 mg/L (z–) and
0.3648 mg/L (z+) in spring and summer, respectively. In the Laurentian Great Lakes, USA, phytoplankton community change-points along
${\rm {NO}}_3^- $ and TP gradients reached 0.285 mg/L (z–), 0.460 mg/L (z+) and
0.0017 mg/L (z–),
0.0053 mg/L (z+) in spring, respectively while they approximately turned to be twice lower than that in summer (
Kovalenko et al., 2017). These thresholds derived from freshwater environments were lower compared with the findings of this study, probably ascribed to the different aquatic environments. In freshwater ecosystems, phytoplankton community generally received nutrients from univariate sources (e.g., agricultural wastes and domestic sewage) within a single watershed despite characterizing with high-level concentrations. On the contrary, in marine systems, especially the coastal waters of northern Zhejiang Province, one of the most severely eutrophic and environmentally heterogeneous areas (
Ye et al., 2017), it may be quite different. Given the complex hydrodynamic conditions, mixed riverine inputs (e.g., Changjiang River, Qiantang River, Cao’e River and Yongjiang River) posed considerable effects upon biological communities. Phytoplankton community in this area was significantly influenced by CDW and Taiwan Warm Current (TWC) in wet seasons (
Jiang et al., 2015;
Zhou et al., 2017a). Moreover, global climate changes continually played an unnegligible role in structuring marine phytoplankton community composition (
Harding et al., 2016;
Conde et al., 2018). These findings implied that phytoplankton community in coastal waters experienced multi-cumulative stresses, which consequently responded more complicated and unpredictable to environmental variability than that in freshwater systems. However, the results of this study did not coincide with a recent study which showed that the thresholds proposed for TN and TP in the region outside Changjiang River Estuary and coastal Zhoushan waters approached to 0.27–0.29 mg/L and 0.023–0.028 mg/L, respectively (
Yang et al., 2019). Differences between the two studies may be attributable to the analytical methods used. In the present study, this study concluded the ecological thresholds by taking into account the biological responses to environmental stresses; while
Yang et al. (2019) applied frequency distribution approach (
US EPA, 2001) to establish nutrient criteria. Despite they yielded numeric nutrient thresholds by collecting long-term dataset in such a complex region in terms of environmental heterogeneity, they merely emphasized on the stressors (concentrations of nutrients) and neglected the effects on responders (biological communities).