An evaluation of long-term physical and hydrochemical measurements at the Sylt Roads Marine Observatory (1973–2019), Wadden Sea, North Sea

Abstract

Abstract. The Sylt Roads pelagic time series covers physical and hydrochemical parameters at five neighboring stations in the Sylt-Rømø Bight, Wadden Sea, North Sea. Since the beginning of the time series in 1973, sea surface temperature (SST), salinity, ammonium, nitrite, nitrate and soluble reactive phosphorus (SRP) were measured twice a week. Other parameters were introduced later (dissolved silicate (Si) – since 1974, pH - since 1979, dissolved organic nitrogen (DON) – since 1996, dissolved organic phosphorus (DOP) – since 2001, chlorophyll a – since 1979, suspended particulate matter (SPM) – since 1975) and in case of dissolved oxygen were already discontinued (1979–1983). In the years 1977, 1978 and 1983 no sampling took place. Since the start of the continuous sampling in 1984, the sea surface temperature in the bight has risen by +1.11 °C, with the highest increases during the autumn months, while the pH and salinity decreased by 0.23 and 0.33 units, respectively. Summer and autumn salinities are generally significantly elevated compared to spring and winter conditions. Dissolved nutrients (ammonium, nitrite, nitrate and SRP) displayed periods of intense eutrophication (1973–1998) and de-eutrophication since 1999. Silicate showed significantly higher winter levels since 1999. Interestingly, phytoplankton parameters did not mirror these large changes in nutrient concentrations, as a seasonal comparison of the two eutrophication periods showed no significant differences with regard to chlorophyll a. This phenomenon might be triggered by an important switch in nutrient limitation during the time series: Until 1998, the phytoplankton was probably primarily limited by silicate, and since 1999 SRP limitation became increasingly important. Repository-Reference: Rick et al. (2017b–e, 2020a–o) doi: 10.1594/PANGAEA.150032, 873549, 873545, 873547, 918018, 918032, 918027, 918023, 918033, 918028, 918024, 918034, 918029, 918025, 918035, 918030, 918026, 918036, 918031.

An evaluation of long-term physical and hydrochemical measurements at the Sylt Roads Marine Observatory (1973–2019), Wadden Sea, North Sea

“…Despite these large changes in nutrient concentrations, phytoplankton parameters such as chlorophyll a (Figs. 3h, 8; Table A1i) or phytoplankton carbon (Rick et al, 2017a) did not shift accordingly, as one would probably have expected (e.g., Cadee and Hegeman, 2002).…”

“…Despite the omission of the biased values (1993)(1994)(1995)(1996)(1997), a t test comparison for all seasons resulted in significantly lower values for the low eutrophication period . This cannot be explained either by lowered plankton biomass (Rick et al, 2017a) or by less sediment input into the water during these years. We assume that a change in the SPM methodology might be the cause.…”

“…Diatoms are the most prominent phytoplankton group in the bight during all seasons Rick et al, 2017a. In addition to diatoms, the prymnesiophyte Phaeocystis globosa Scherffel, 1899 may add substantially to the photosynthetic biomass in late spring and early summer (Rick et al, 2017a).…”

“…Diatoms are the most prominent phytoplankton group in the bight during all seasons Rick et al, 2017a. In addition to diatoms, the prymnesiophyte Phaeocystis globosa Scherffel, 1899 may add substantially to the photosynthetic biomass in late spring and early summer (Rick et al, 2017a). During the period of high phosphorus and nitrogen loads (1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998), initial winter silicate (Fig.…”

“…Mostly on a twiceweekly basis, microplankton abundance and related biomass parameters, such as plankton biovolume and carbon, were recorded. All data until 2013 are compiled in the PANGAEA repository (Rick et al, 2017a).…”