Description: Facing the predicted rise in global sea level, sandy shorelines are under increasing pressure. In order to counteract the loss of material at eroding coastlines, beach nourishment is considered to be an environmentally friendly approach worldwide. This has resulted in a rising demand for aggregates, which are frequently extracted from the seafloor near the coast. In order to explore the long- and short-term morphological changes of such mining on the seabed, the largest extraction area in the German Bight (Westerland Dredging Area, established in 1984) was investigated in this study. Several measurement campaigns were conducted between the years 1994 and 2017 using a set of hydroacoustic techniques. The measurements revealed that up to 20-m-deep pits with diameters of more than 1 km were dredged into the seafloor. The depressions caused by this sand mining are still detectable more than 30 years later. Because of slope failures that mainly consist of fine sand, the formerly steep rims at fresh dredging pits smoothed within a few months. However, after approximately 1 year, muddy sediments dominated the deposition. Since the sedimentation rates are slow, a complete backfill of the post-dredging pits is likely to take many decades. A natural regeneration towards the former seafloor conditions is only visible at the shallow rims of the oldest dredging pits.

Description: To appreciate coastal trophodynamics, it is necessary to understand the dynamics and control of the spring and late summer/autumn phytoplankton blooms. Classically mesozooplankton are considered as main players in these blooms. Microzooplankton likely also are important in these dynamics, but their role is poorly understood. Critically, due to their rapid generation times, microzooplankton may exhibit rapid shifts during blooms. Through field sampling and rate measurements (dilution experiments) in a well-studied temperate coastal ecosystem (Helgoland, southern North Sea) we ask if there are differences in the trends exhibited between and within the spring and late summer/autumn blooms. To achieve this, we examined early, mid and late bloom periods in both seasons. We found 1) a shift in trophic composition during both blooms, with a trend from strongly autotrophic mixotrophs (e.g.Mesodinium) to mixotrophs and then towards heterotrophs; 2) an increase in intraguild predation at the end of the blooms; and 3) although microzooplankton were major consumers of the spring bloom (grazing coefficientg: 0.23-0.25 d(-1); daily percent loss of productionP(p): 36-47%), they were unlikely to control it, while in contrast, microzooplankton appeared to play a major role in controlling the late summer/autumn bloom (grazing coefficientg: 0.14-1.53 d(-1); daily percent loss of productionP(p): 24-103%). In doing so, we suggest that any simplifications that consider these seasonal blooms to be relatively homogeneous and similar will lead to substantial errors in the assessment of coastal trophodynamics.

Description: Analysing long-term diatom data from the German Bight and observational climate data for the period 1962–2005, we found a close connection of the interannual variation of the timing of the spring bloom with the boreal winter atmospheric circulation. We examined the fact that high diatom counts of the spring bloom tended to occur later when the atmospheric circulation was characterized by winter blocking over Scandinavia. The associated pattern in the sea level pressure showed a pressure dipole with two centres located over the Azores and Norway and was tilted compared to the North Atlantic Oscillation. The bloom was earlier when the cyclonic circulation over Scandinavia allowed an increased inflow of Atlantic water into the North Sea which is associated with clearer, more marine water, and warmer conditions. The bloom was later when a more continental atmospheric flow from the east was detected. At Helgoland Roads, it seems that under turbid water conditions (= low light) zooplankton grazing can affect the timing of the phytoplankton bloom negatively. Warmer water temperatures will facilitate this. Under clear water conditions, light will be the main governing factor with regard to the timing of the spring bloom. These different water conditions are shown here to be mainly related to large-scale weather patterns. We found that the mean diatom bloom could be predicted from the sea level pressure one to three months in advance. Using historical pressure data, we derived a proxy for the timing of the spring bloom over the last centuries, showing an increased number of late (proxy-) blooms during the eighteenth century when the climate was considerably colder than today. We argue that these variations are important for the interpretation of inter-annual to centennial variations of biological processes. This is of particular interest when considering future scenarios, as well to considerations on past and future effects on the primary production and food webs.

Description: In recent regime shift analyses, the phytoplankton compartment of the marine food web was essentially represented by phytoplankton color or chlorophyll concentration. A detection of changes directly at the species level is highly desirable. The Helgoland Roads data series, a collection of high frequency long-term time series comprising biological and physico-chemical components of the southern North Sea, allow such an investigation at the level of single species. Aiming at a detection and characterization of habitat and community changes in the observation period (1962 until the end of 2008), we selected six species as representatives of certain classes, for example, benthic or neritic species, and applied a combination of novel analysis methods—a fitness-based analysis of the realized niche, a bloom-triggered averaging and a Markovian analysis of co-occurrence and succession patterns—to related abundance time series and concurrent environmental parameter time series. We found a general trend toward enlargement of niche size and shifts of the niche position, interesting salinity patterns around bloom events of two species, and statistically highly significant changes of a phytoplankton community segment after 1965 and after 1998. Interpreting our observations in ecological terms leads to the formulation of testable hypotheses.

Description: Vibrio species are ubiquitously distributed in marine waters all over the world. High genome plasticity due to frequent mutation, recombination, and lateral gene transfer enables Vibrio to adapt rapidly to environmental changes. The genus Vibrio comprises several human pathogens, which commonly cause outbreaks of severe diarrhea in tropical regions. In recent years, pathogenic Vibrio emerged also in coastal European waters. Little is known about factors driving the proliferation of Vibrio spp. in temperate waters such as the North Sea. In this study a quantification of Vibrio in the North Sea and their response to biotic and abiotic parameters were assessed. Between January and December 2009, Vibrio at Helgoland Roads (North Sea, Germany) were quantified using fluorescence in situ hybridization. Vibrio numbers up to 3.4× 104 cells×mL−1 (2.2% of total microbial counts) were determined in summer, but their abundance was significantly lower in winter (5×102 cells×mL−1). Correlations between Vibrio and nutrients (SiO2, PO4 3−, DIN), Secchi depth, temperature, salinity, and chlorophyll a were calculated using Spearman rank analysis. Multiple stepwise regression analysis was carried out to analyze the additive influence of multiple factors on Vibrio. Based on these calculations, we found that high water temperature and low salinity best explained the increase of Vibrio cell numbers. Other environmental parameters, specially nutrients and chlorophyll a, also had an influence. All variableswere shown to be subject to the overall seasonal dynamics at Helgoland Roads. Multiple regression models could represent an efficient and reliable tool to predict Vibrio abundances in response to the climate change in European waters.