Synthesis of climate relevant results from selected monitoring programs in the coastal zone. Part 2: Quantitative analyses (original) (raw)

Influence of Riverine Input on Norwegian Coastal Systems

Frontiers in Marine Science

Coastal ecosystems are of high ecological and socioeconomic importance and are strongly influenced by processes from land, sea, and human activities. In this study, we present physical, chemical, and biological observations over two consecutive years from three study regions along the Norwegian coast that represent a broad latitudinal gradient in catchment and oceanographic conditions (∼59-69 • N): outer Oslofjord/southern Norway, Runde/western Norway, and Malangen/northern Norway. The observations included river monitoring, coastal monitoring, and sensor-equipped ships of opportunity ("FerryBox"). The riverine discharge and transports were an order of magnitude higher, and the spatiotemporal extent of this freshwater influence was larger in the coastal region in southern Norway, compared to western and northern Norway. The southern Norway coastal waters had consistently high dissolved organic carbon (DOC) and chromophoric dissolved organic matter (cDOM) fluorescence year-round, connected to the large influence of local riverine input and likely also advected riverine runoff and mixing with water masses from the southern North Sea and Baltic Sea. Meanwhile, the western and northern study regions were more sheltered and characterized by more episodic riverine input of freshwater, DOC, cDOM, and nutrients. The timing of the spring phytoplankton bloom in all three regions generally preceded the periods of high riverine input, which suggested that while the winter nutrient reserve was sufficient to fuel the spring bloom, the input of nutrients during the spring flood could sustain the spring bloom or the input of suspended matter, and DOC/cDOM could result in light limitation of the bloom. This article summarizes the impact of riverine input on three diverse coastal systems in terms timing and duration, as well as the potential consequences for ecosystem function especially as related to rising terrestrial organic matter input into coastal regions over the last decades and the projected increase due to climate change.

Soft bottom benthos and responses to climate variation and eutrophication in Skagerrak

Journal of Sea Research, 2018

Skagerrak has been subject to several anthropogenic influences over the past decades, with climate change and eutrophication being considered as the most serious and large-scale disturbance factors. The present study reports monitoring data from six soft bottom stations in 50-380 m depth at the Norwegian Skagerrak coast aimed at investigating to which degree changes in environmental conditions have affected species communities and diversity. Sampling was carried out yearly in the period 1990-2010. Links between benthic community patterns and climate factors and physicochemical variables from the water mases were examined using uni-and multivariate statistical methods. Throughout the period species richness gradually increased. Although all stations showed distinct species assemblages, the community composition gradually changed towards increased importance of sensitive small molluscs and tube-building annelids concurrent with a general temperature increase and reduction of nutrients in the water masses. The trend was largely similar over the stations, indicating that large-scale changes in the Skagerrak water masses were driving factors compared to possible influences from local sources. The faunal changes during the study period thus indicate an improved status of the soft bottom benthos, which possibly could be related to a reduction in the eutrophication. On a shorter scale, species richness was found to vary in relation to North Atlantic Oscillation (NAO) Index in the previous year (decline), nutrient concentrations in spring (decline), and winter water temperature (incline). Keywords: Soft bottom benthos, species richness, climate change, eutrophication, time-seriessubtidal hard bottom communities, as well as for climatic factors, nutrients, particle loading and microalgae in the pelagic. Thus, the programme covers a multitude of environmental and biological parameters from shallow to deeper areas in the Skagerrak and eastern North Sea. The main aim of the programme has been to reveal possible effects of eutrophication and climate change on the coastal ecosystems. It has been an important part of the project to distinguish between the effects from long-distance transported substances and local sources. Environmental management needs better information about complex ecosystem dynamics (Frid et al. 2005), and about the single and interactive effects of disturbances such as eutrophication and climatic variation on marine ecosystems. The aim of the present work is to examine the development of the coastal soft bottom communities in the Skagerrak within the period 1990-2010 and the influence of eutrophication and climatic variation. Specifically, spatial and temporal changes in water temperature, salinity, nutrients, and suspended particles are related to species richness, diversity, community structure and community functioning. The effects on shallow water hard bottom systems for the same time period have been reported by Norderhaug et al. (2015). 2. MATERIALS AND METHODS 2.1. Sea area characteristics The Skagerrak is a part of the North Sea situated between the southeast coast of Norway, the southwest coast of Sweden, and the Jutland peninsula of Denmark. It connects the main North Sea and the Kattegat sea area, which leads to the Baltic Sea (Figure 1). It is a hydrodynamically complex area, where water masses from the North Sea and the shallow, brackish Kattegat meet and mix (Figure 1). The coastal water along the Norwegian Skagerrak coast is basically a mixture of two water masses; Atlantic water and freshwater. Most of the freshwater comes from three sources; local runoff to the coast, the Baltic Sea and the large rivers draining to the southern part of the North Sea. These water masses combine to form the Norwegian Coastal Current. The Coastal Current and thereby Skagerrak receives large regional nutrient inputs from European rivers (Aure & Magnusson 2008). The mean annual freshwater supply to the Skagerrak from the Baltic Sea and the Kattegat is estimated to ca. 215 000 m 3 s-1 , and in addition, a large fraction of the 4 500 m 3 s-1 of continental river discharge to the North Sea passes through the area (Aure et al. 1998). Particularly water from the German Bight strongly influences the water quality. This water contributes to approximately 75% of nitrate and 40% of phosphate in the Coastal Current, respectively, but in the period 1990-1995, when discharges from European rivers reached a maximum level, the contribution was approximately 83% and 48%, respectively (Aure & Magnusson 2008). Strong management effort has lead to an improvement in the water quality, although the current levels still are considerable higher than during earlier periods (1965-1980) (Norderhaug et al. 2011). Notably, in contrast to declining nutrient concentrations, the concentrations of carbon and nitrogen in seston, dissolved organic nitrogen and the estimated fraction on non-autrophic material have been found to undergo a rapid increase between 1998 and 2000, and have remained at a higher level since (Frigstad et al. 2013). This increase is probably caused by increased inputs of terrestrial-derived, humic material due to an increased freshwater runoff (Frigstad et al. 2013). Supplement 2. Box (interquartile range) and whisker (extends to the most extreme data points) plots of yearly variation in measured environmental variables used in the soft bottom dataset, averaged for all stations. Symbols indicate significant levels at <0.001 (***), <0.01 (**), <0.05 (*), and <0.1 (•) for the regression through time for each environmental variable. Pelite content is measured as % particles < 0.063 mm, temperature (T) is given in °C, salinity (Sal) in ppt, Total Organic Carbon (TOC) in mg g-1 whereas all nutrients, i.e. total phosphor (TotP), phosphate (PO 4), total nitrogen (TotN), nitrate + nitrite (NO 3 +NO 2), Particulate Organic Carbon (POC), and Nitrogen (PON) are given in µM.

Coastal eutrophication and trend reversal: A Danish case study

Limnology and Oceanography, 2006

In the past 2 decades significant measures have been taken to reduce nitrogen and phosphorus discharges from Denmark by 50% and 80%, respectively. These nutrient reduction targets now appear within reach after several consecutive reduction measures are fully implemented, particularly toward diffuse discharges, and reduced nutrient concentrations are beginning to be observed in estuaries and the Danish straits. Phosphorus concentrations have declined by 22% to 57% from the early 1990s, mainly owing to improved treatment of urban and industrial wastewater. Changes in nitrogen concentrations, following reduction measures toward diffuse sources, were more recent and partly masked by large interannual variations in freshwater discharge. The response in marine nitrogen concentrations was delayed relative to the decline in riverine concentrations, most likely owing to large internal loading from the sediments. Two consecutive dry years appeared to be the triggering mechanism for nitrogen concentrations to decline. In the last 5 yr, nitrogen levels in estuaries and coastal waters have decreased up to 44% when interannual variations in freshwater discharge were accounted for. These first signs of environmental recovery were most pronounced in estuaries and coastal waters but also were apparent in open waters of the Kattegat, the Sound, and the Belt Sea. This case study is the first to document significant decreases in nutrient concentrations on a large regional scale resulting from an active management strategy to reduce nutrients from both diffuse and point sources.