Modeling of intra-annual abundance distributions: Constancy and variation in the phenology of marine phytoplankton species over five decades at Helgoland Roads (North Sea)

Annually recurring environmental processes such as the cycle of temperature and light drive the phenology of marine plankton populations. Improved knowledge about the homogeneity and amplitude of the phenological response of phytoplankton to climate change is essential for an assessment of ecological consequences on the marine ecosystem. We analyzed phenological variability of 21 phytoplankton species monitored work-daily at Helgoland Roads from 1962-2015. We used a function of “Weibull”-type to estimate phenological dates of species-specific abundance peaks. The combination of derived dates and peak abundances formed the basis for the analyses of long-term changes in phenological time slots and associated environmental conditions. Species-specific preferences in combination with seasonally varying environmental trends resulted in a complex pattern of phenological long-term response. Phenological trends showed both constant occurrence and shifts to an earlier or later occurrence. Co-occurring phytoplankton species were shown to exhibit different phenological trends even within identical time slots. Differences in species-specific trends in timing also reflected the seasonally varying shifts in water temperature ranges due to warming. In spring and summer, the main patterns of common variability in timing were associated with different abiotic and biotic drivers. The majority of species showed more narrow time slots related to the occurrence of higher peaks. Considering the variation of species occurrence in their “typical” time window provided insight in terms of assigning the effect of environmental drivers on inter-annual phenological variation. Phytoplankton species with similar long-term trends in timing (days) showed different trends in biomass, i.e. the phenological changes resulted from different ecological responses to environmental change. The local character of environmental trends at Helgoland underpins the limits for comparison of findings between different measuring sites or wider areas, such as the North Sea. The study emphasizes the benefit and necessity of a highly resolved phytoplankton record for a true understanding of long-term ecological changes in a highly dynamic marine environment such as the North Sea

Klimawandel und Plankton

Ein besseres Verständnis der Auswirkungen des Klimawandels auf die marinen Ökosysteme ist für den Menschen sowohl aus ökologischer wie auch ökonomischer Sicht von herausragender Bedeutung. Die Reaktion von Phyto- und Zooplanktern auf klimatische Veränderungen spielt aufgrund ihrer Funktion als Primär- und Sekundärproduzenten im marinen Nahrungsnetz eine zentrale Rolle in diesem Prozess. Die Erwärmung der oberflächennahen Schichten in Ozeanen und Randmeeren hat ökologische Veränderungen in Form von Verschiebungen in den biogeographischen und zeitlichen Verteilungsmustern von planktischen Organismen zur Folge. Indirekte Temperatureffekte können u.a. aus einer verstärkten Stratifizierung, welche den vertikalen Austausch von Nährstoffen und damit ihre Verfügbarkeit für das Phytoplankton in der euphotischen Zone reduziert, resultieren. Unterschiede in der phänologischen Reaktion von Phyto- und Zooplanktern hinsichtlich gestiegener Wassertemperaturen können zur innerjährlichen Entkopplung funktionaler Verbindungen zwischen beiden Gruppen führen, mit Folgen auch für die Nahrungsverfügbarkeit und -qualität von Fischen. Der Anstieg der atmosphärischen Kohlendioxid-Konzentration (CO2) führt zu einer Versauerung der Meere und dadurch zu Veränderungen der chemischen Zusammensetzung des Meerwassers. Eine Abnahme des pH-Wertes hat u.a. potentiell negative Implikationen für kalzifizierende, planktische Organismen. Klimatische Effekte auf marine Ökosysteme können sich lokal und regional unterschiedlich manifestieren und, etwa in Küstenregionen, durch anthropogene Einflüsse (z.B. Nährstoffeinträge) überlagert werden. Biologisch-physikalische gekoppelte Modelle können einen Beitrag liefern, die Auswirkungen klimatischer Änderungen auf marine Ökosysteme besser zu verstehen und potentielle, zukünftige Entwicklungen abzuschätzen

Extreme Flood Impact on Estuarine and Coastal Biogeochemistry: the 2013 Elbe Flood

Within the context of predicted and observed increase in droughts and floods with climate change, large summer floods are likely to become more frequent. These extreme events can alter typical biogeochemical patterns in coastal systems. The extreme Elbe River flood in June, 2013 not only caused major damages in several European countries, but also generated large scale biogeochemical changes in the Elbe Estuary and the adjacent German Bight. Due to a number of well documented and unusual atmospheric conditions, the early summer of 2013 in Central and Eastern Europe was colder and wetter than usual, with saturated soils, and higher than average cumulative precipitation. Additional precipitation at the end of May, and beginning of June, 2013, caused widespread floods within the Danube and Elbe Rivers, as well as billions of euros in damages. The floods generated the largest summer discharge on record within the last 140 years. The high-frequency monitoring network in the German Bight available within the Coastal Observing System for Northern and Arctic Seas (COSYNA) captured the flood influence on the German Bight. Monitoring data from a FerryBox station in the Elbe Estuary (Cuxhaven) and from a FerryBox platform aboard the M/V Funny Girl Ferry (traveling between Büsum and Helgoland) documented the salinity changes on the German Bight, which persisted for about 2 months after the peak discharge. The flood generated a large influx of nutrients, dissolved and particulate organic carbon on the coast. These conditions subsequently led to the onset of a chlorophyll bloom within the German Bight, observed by dissolved oxygen supersaturation, and higher than usual pH in surface coastal waters. The prolonged stratification also led to widespread bottom water dissolved oxygen depletion, unusual for the south eastern German Bight in the summer

Long-term changes (1958-2004) in the hydro-climatic environment of the southern North Sea and consequences for the interpretation of observations on the island Helgoland (German Bight)

A 47-yr (1958-2004) model simulation has been analyzed to identify changes of the southern North Sea hydrodynamic regime in the past. A time series analysis revealed time points of changes in volume transports which correspond to recently described changes in the North Sea ecosystem (‘regime shifts’). The strengths of these hydrodynamic changes are shown to vary on a regional scale. Being interested in the analysis of long-term time series (starting in 1962) of hydrophysical and biological parameters measured at the island of Helgoland (54°11.3’ N, 7°54.0’ E, German Bight) we studied seasonal and interannual variations of the North Sea hydrodynamic regime with respect to their effects on the Helgoland area. An Empirical Orthogonal Function (EOF) analysis of spatial patterns of passive tracer Lagrangian transports has been carried out to describe the temporal variability of water mass advection. It could be shown that the southern inflow to the North Sea (via the English Channel) has strengthened since the end of the 70s and that the degree of its impact in the Helgoland area has changed. These regional effects of large-scale hydro-meteorological changes are also discussed in terms of its connection with variations of the NAO index

Mean spring conditions at Helgoland Roads, North Sea: Graphical modeling of the influence of hydro-climatic forcing and Elbe River discharge

We analyze inter-annual changes of marine observations at Helgoland Roads (nitrate, phosphate, salinity, Secchi depth) in relation to hydro-climatic conditions and Elbe River discharge as potential drivers. Focusing on mean spring conditions we explore graphical covariance selection modeling as a means to both identify and represent the structure of parameter interactions. While river discharge is able to modify spatial distributions and related gradients in the station's vicinity, atmospherically forced regional transport patterns govern the time dependent local conditions the station is actually exposed to. A model consistent with the data confirms the interplay of the two forcing factors for observations at station Helgoland Roads. Introducing water temperature as a third predictor of inter-annual variability does not much improve the model. Comparing a Helgoland Roads dependence graph with corresponding graphs for other stations or related model simulations, for instance, could help identify differences in underlying mechanisms without referring to specific realizations of external forcing. With regard to prediction, supplementary numerical experiments reveal that imposing constraints on parameter interactions can reduce the chance of fitting regression models to noise

Plankton phenology on the long–term scale: consistency and variation observed at Helgoland Roads

The annual and periodical recurrence of events and processes in marine plankton populations plays a central role in pelagic food webs. Phytoplankton in shallow coastal seas is subject to high seasonal variability which makes analyses of phenological changes and their classification as imprints of climatic changes difficult. The extent of natural variability relative to trend-like dynamics can only be determined if phytoplankton observations are both regular and long-term. We analyze phenological variability of diatom species monitored at Helgoland Roads during the period 1962-2013. We derive temporal indices based on the cumulative percentage of annual abundance to approximate the beginning and the end of individual annual growth periods and analyze their relationships to abiotic and biotic parameters. Timing of phytoplankton species is highly variable and only very few species exhibit significant trends in phenological characteristics over the entire period. Phytoplankton species can exhibit a wide range of environmental conditions associated with the beginning of growth. As a result, strong fluctuations in winter/spring temperatures do not automatically lead to changes in timing dates. Most species exhibit a broader timing window at the beginning of the growth period than at the end. Short phases of rapidly changing light conditions during spring and autumn seem to play an important role as a sort of ´climatic boundary` for phytoplankton seasonality. We will put the phenological response in context to long-term trends in abiotic determinants (e.g. temperature) observed at Helgoland Roads and discuss the potential of making assessments about consequences for the marine ecosystem