Ocean Variability of Helgoland Roads Time Series Data Using Spectral Analysis

The understanding of coastal ecosystems regarding variability and resilience under climatic and anthropogenic forcing is reliant upon long-term ecological records. We examined the Helgoland Roads time series (1968–2017), which includes temperature, salinity, nutrients (nitrate, phosphate), and biological parameters (diatoms and Acartia spp.). We applied autocorrelation, multi-taper spectral analysis, wavelet and cross-wavelet transforms to identify dominant temporal patterns and scale-dependent interactions. Sea surface temperature shows consistent long-term warming, and sub-decadal (2–3 year) and decadal (7–8 year) oscillations reflect coherent patterns with the North Atlantic Oscillation and Arctic Oscillation. Salinity varied in antiphase to Elbe River discharge at 6–7 year scales, reflecting control of seasonal, riverine freshwater and salinity scenarios. Nutrients, both as declining long-term trends (particularly phosphate), are associated with seasonal to multi-year variability linked to episodic discharge events. Biological parameters had strong annual periodicities reflective of bloom cycles, but also variability above the annual limit. Diatoms responded to climatic, nutrient, biological responses at the 3–5 year scale associated with this ecological context, particularly nitrate and phosphate; Acartia (spp) respond to temperature, salinity and resource availability (diatoms) reflecting climate-nutrient-trophic linkages. This study indicate that Helgoland Roads is represented as a multi-scale, non-stationary system, in which climate variability, riverine input and ecological linkages are cascaded down to physical and chemical processes that structure biological communities. Spectral methods reveal scale-dependent synchrony and highlight risks of trophic mismatch under climate change, emphasizing the importance of sustained, high-frequency monitoring.