Jellyfish blooms restructure plankton dynamics and trophic linkages in coastal waters

Jellyfish blooms are increasing globally in frequency and intensity, introducing complex ecological interactions, yet the mechanisms by which they alter ecosystem structure remain poorly characterized due to a lack of sustained field observations.

We conducted targeted time-series observations in Jiaozhou Bay—a representative coastal ecosystem experiencing Aurelia coerulea blooms—tracking temporal responses of plankton communities and trophic interactions to jellyfish population fluctuations. A total of 36 surveys were conducted over three years from 2021 to 2023, yielding comprehensive datasets on biological variables (jellyfish biomass, zooplankton abundance and composition, phytoplankton abundance and composition), environmental factors (temperature, salinity, pH), and nutrient concentrations in the study area. Additionally, we obtained datasets on the gastric content composition of field-collected A. coerulea through stomach dissection analysis. We further quantified the dual roles of jellyfish as top-down predators and bottom-up nutrient regenerators, linking bloom dynamics to changes in plankton structure and biogeochemical cycling. Our results revealed that during periods of high A. coerulea aggregation, the structure of the plankton community changed markedly, characterized by a sharp decline in zooplankton coupled with the proliferation of phytoplankton. This top-down feeding effect triggered potential cascading impacts, decoupling trophic pathways by weakening zooplankton-mediated energy transfer and releasing phytoplankton from grazing pressure. Concurrently, jellyfish contributed to biogeochemical processes by releasing bioavailable ammonium and phosphate, with phosphate regeneration playing a particularly critical role in stimulating primary production.

Methods
1. Study area and investigated time

Jiaozhou Bay, located in the Yellow Sea, China, is a shallow, semi-enclosed coastal system that is heavily influenced by anthropogenic activities The study area (mean depth of ~4 m) has been established as a reference site for long-term ecological monitoring (Sun et al., 2011). It also serves as a nursery ground for Aurelia coerulea and experiences recurrent seasonal jellyfish blooms (Wang et al., 2021), making it representative for assessing the ecological impacts of jellyfish blooms in Jiaozhou Bay. From 2021 to 2023, we implemented a fixed site temporal monitoring program to control spatial variability, synchronously capturing coupled measurements of environmental conditions, jellyfish biomass, plankton communities, and nutrient dynamics. Given the strong seasonality and interannual variability of A. coerulea blooms (Wang and Sun, 2015), multiyear, site-specific monitoring allowed us to capture temporal ecological responses while eliminating spatial heterogeneity in plankton and nutrient distributions (Wang et al., 2024). By comparing seasonal variations between bloom and non-bloom years, we examined jellyfish bloom-associated shifts in ecosystem structures under consistent spatial conditions.

Strobilation of A. coerulea polyps begins in April, with young medusae appearing in May; sexual maturity is reached by mid-summer, followed by a population decline in August (Wang and Sun, 2015). Our surveys spanned April to September each year, comprising 12 sampling events annually, with intensified sampling (3 time points per month) during peak bloom periods (June-August). Based on monitoring data, we classified 2021-2022 as non-bloom years and 2023 as a jellyfish bloom year, with mid-June to mid-July designated as the jellyfish bloom stage (abundance exceeded 1 ind m⁻³) for interannual comparisons.

2. Hydrological and biological parameters

2.1 Hydrological parameters

Hydrological parameters, including temperature (°C), salinity, and pH, were measured in triplicate at each sampling using a AAQ1183-1F CTD (Alec Electronics Co., Japan).

2.2 Population ofAurelia coerulea

The abundance of medusae (ind m⁻³) was estimated through visual surveys following Yoon et al. (2007). All surveys were carried out in the morning. The medusae were then randomly collected using a hand net (1-mm mesh) to measure bell diameter (D, mm). Biomass (g m⁻³) was calculated using the regression equation established by Uye and Shimauchi (2005), relating bell diameter to wet weight (WW, g).

2.3 Nutrients

Triplicate water samples for inorganic nutrient analysis (silicate (Si), phosphate (P), nitrate (NO₃⁻), nitrite (NO₂⁻), and ammonium (NH₄⁺)) were collected and filtered through a 0.45-μm mixed cellulose ester filter (diameter: 33 mm). The filtered samples were stored in high-density polyethylene bottles and frozen at -20°C for later analysis. Prior to sampling, storage bottles were acid-washed with 10% hydrochloric acid and rinsed with ultrapure water. Nutrient concentrations were determined using flow injection analysis (Technicon AA3 Auto-Analyzer; Bran+Luebbe Co., Germany).

2.4 Chlorophyll a

For chlorophyll a (Chl a) analysis, 1-liter triplicate water samples were collected at each sampling event. The samples were filtered through cellulose ester filters (pore size: 0.45 μm), and 0.2 ml of MgCO₃ was added to each filter to prevent Chl a decomposition. The filters were then frozen at 4 °C and stored in darkness. Chl a was extracted from the filters using 90% acetone, following the method described by Lorenzen (1967), and the concentrations were obtained by a spectrophotometric method (TU-1810 spectrophotometer).

2.5 Plankton assemblages

Phytoplankton (20–160 μm) were analyzed from triplicate 1-L water samples. The samples were first filtered through a 160-μm sieve and immediately preserved with Lugol iodine acid (final concentration 1%) in brown plastic bottles. Phytoplankton were identified and counted under an inverted microscope (Olympus IX71), classified into diatoms and dinoflagellates, and reported as abundance per liter (ind L⁻¹). Zooplankton (>160 μm) were collected by vertical tows from bottom to surface using a 160-μm mesh plankton net (mouth area: 0.08 m²) and preserved in 5% neutral formalin. Organisms were classified into copepods, small medusae (siphonophores and hydromedusae), chaetognaths, tunicates, and other taxa (for example, crab and shrimp larvae, bivalve veligers). Abundance was determined using a stereomicroscope (Nikon SMZ745, Tokyo, Japan) and expressed per cubic meter (ind m⁻³).

3. Gut content analysis

A total of 62 undamaged** medusae were collected throughout the study period 2023 for gut content analysis (GCA). The medusae were immediately preserved in 5% neutral formalin in 20-μm filtered seawater. Prior to GCA, each medusa was gently rinsed with ultrapure water to remove any externally attached prey, and bell diameters were recorded. Gastric pouches were exposed by carefully incising around the gut on the side of the subumbrella. The pouches were rinsed with 20-μm filtered seawater, and the released prey were collected in a 5-L beaker (Wang et al., 2023). Liquid samples were also collected to check for prey items. The prey were concentrated using a 20-μm mesh sieve, then identified and counted under a dissecting microscope (Nikon SMZ745, Japan).

References

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Wang, P., Zhang, F., Guo, D., & Sun, S. (2021). Diets and seasonal ingestion rates of Aurelia coerulea (Cnidaria: Scyphozoa) polyps by in situ feeding experiments in Jiaozhou Bay, China. Frontiers in Marine Science, 8.

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