Spring blooms in the Baltic Sea have weakened but lengthened from 2000 to 2014

Abstract

Abstract. Phytoplankton spring bloom phenology was derived from a 15-year time series (2000–2014) of ship-of-opportunity chlorophyll a fluorescence observations collected in the Baltic Sea through the Alg@line network. Decadal trends were analysed against inter-annual variability in bloom timing and intensity, and environmental drivers (nutrient concentration, temperature, radiation level, wind speed).Spring blooms developed from the south to the north, with the first blooms peaking mid-March in the Bay of Mecklenburg and the latest bloom peaks occurring mid-April in the Gulf of Finland. Bloom duration was similar between sea areas (43 ± 2 day), except for shorter bloom duration in the Bay of Mecklenburg (36 ± 11 day). Variability in bloom timing increased towards the south. Bloom peak chlorophyll a concentrations were highest (and most variable) in the Gulf of Finland (20.2 ± 5.7 mg m−3) and the Bay of Mecklenburg (12.3 ± 5.2 mg m−3).Bloom peak chlorophyll a concentration showed a negative trend of −0.31 ± 0.10 mg m−3 yr−1. Trend-agnostic distribution-based (Weibull-type) bloom metrics showed a positive trend in bloom duration of 1.04 ± 0.20 day yr−1, which was not found with any of the threshold-based metrics. The Weibull bloom metric results were considered representative in the presence of bloom intensity trends.Bloom intensity was mainly determined by winter nutrient concentration, while bloom timing and duration co-varied with meteorological conditions. Longer blooms corresponded to higher water temperature, more intense solar radiation, and lower wind speed. It is concluded that nutrient reduction efforts led to decreasing bloom intensity, while changes in Baltic Sea environmental conditions associated with global change corresponded to a lengthening spring bloom period.

Contrasting seasonality in optical-biogeochemical properties of the Baltic Sea

“…Consequently, the spring bloom has an intense but short-lived impact on the optical properties of the surface layer [54,55]. In contrast, thermal stratification in summer causes the summer particle population to linger in surface waters.…”

Shifting Diatom—Dinoflagellate Dominance During Spring Bloom in the Baltic Sea and its Potential Effects on Biogeochemical Cycling

“…At present, freshwater induced stratification is important for the initial start of the spring bloom, but thermal stratification may become more important in the future (Hordoir and Meier, 2012). Warming of the Baltic Sea has already caused temporal shifts in the phytoplankton distribution during the highly productive spring, with earlier and more prolonged spring bloom (Groetsch et al, 2016;Kahru et al, 2016). In addition, longterm monitoring data suggest that the phytoplankton community is changing during spring in some areas of the Baltic Sea from diatom to dinoflagellates dominance , as a consequence of the ongoing climate change (Klais et al, 2013).…”

Phenology and time series trends of the dominant seasonal phytoplankton bloom across global scales

“…As an example, in the Baltic Sea, investigators found that bloom duration has increased in recent years and associated this change in bloom dynamics with increasing water temperature and declining wind stress, which they attributed to global climate change (Groetsch, Simis, Eleveld, & Peters, 2016). As climate systems shift in response to anthropogenic forcing, there is a need to understand their impact on bloom dynamics both retrospectively and in a forecasting context.…”

“…As climate systems shift in response to anthropogenic forcing, there is a need to understand their impact on bloom dynamics both retrospectively and in a forecasting context. As an example, in the Baltic Sea, investigators found that bloom duration has increased in recent years and associated this change in bloom dynamics with increasing water temperature and declining wind stress, which they attributed to global climate change (Groetsch, Simis, Eleveld, & Peters, 2016). Change in climate conditions may act to modify blooms through the direct effects of nutrient supply and grazing; additionally, changing distributions of parasites and viruses associated with climate change will be likely to play a larger role in the dynamics of blooms and the nature of fixed carbon available to primary grazers (Frenken et al, 2016).…”