Finnish Rivers Research Articles

Changes in the discharge regime of Finnish rivers

Study regionFinland divided into three subregions, each representing different environmental conditions. Study focusThis study investigates long-term changes in unregulated river discharge. Trends in high- and low-flow event volumes, magnitudes, timings, and frequencies are analysed across 36 gauging stations in 19 watershed areas from 1911 to 2021. The average measurement period for discharge in the stations is 60 years, with over 765,000 daily records examined statistically. New hydrological insights for the regionHigh-flow events show advancing timings and decreasing magnitudes, notably in the coastal region and less so in the north. These events, occurring from 6 to 68 days earlier in 21 stations, now in the late winter and early spring, align with increasing spring low-flow volumes. On a monthly scale, a trend of rising volume magnitude is observed in late autumn, winter, and early spring, especially in Northern Finland's rivers. High flows during autumn and winter occur 30 to 60 days later in 8 stations. Changes in the monthly mean volumes were found in 30 stations, suggesting a redistribution of annual volumes across a broader time period, while the overall annual volumes have remained relatively unchanged. This underscores the complexity of hydrological patterns, emphasizing the need to consider total volumes and their temporal distribution in analyses. The findings enhance understanding of current changes and align with findings in the boreal-subarctic area.

An Eoarchean continental nucleus for the Fennoscandian Shield and a link to the North Atlantic craton

Abstract Enabling the build-up of continental crust is a vital step in the stabilization of cratonic lithosphere. However, these initial crustal nuclei are commonly either destroyed by recycling or buried by younger rocks. In the Fennoscandian Shield, the oldest rocks are ca. 3.5 Ga, but ca. 3.7 Ga inherited and detrital zircons suggest the presence of an older, unexposed crustal substrate. We present U-Pb, O, and Hf isotope data from detrital zircons of three major Finnish rivers as well as zircon O and Hf isotope data from previously dated rocks of the Archean Suomujärvi and Pudasjärvi complexes, central Finland. Combined, these data indicate a previously unidentified ca. 3.75 Ga crustal nucleus in the Fennoscandian Shield. This adds to the growing number of Eoarchean nuclei recognized in Archean terranes around the globe, highlighting the importance of such nuclei in enabling the growth of continental crust. The isotope signatures of the Fennoscandian nucleus correlate with equivalent-aged rocks in Greenland, consistent with a common Eoarchean evolution for Fennoscandia and the North Atlantic craton.

A century of variations in extreme flow across Finnish rivers

River flow in cold climates is known to be one of the hydrological systems most affected by climate change, playing a central role in the sustainability of downstream socio-ecological systems. Numerous studies on the temporal and spatial variations of streamflow characteristics have been done, and a comprehensive study on the variation of hydrologic extremes is becoming increasingly important. This study evaluated the long-running changes in the magnitude, time, and inter-annual variability of hydrologic extremes, including high and low flow in 16 major Finnish rivers. We applied four new hydrologic extreme indices for summer–winter low flow ratio, spring-absolute high flow ratio, time-to-peak index, and increasing rate index during the snowmelt period to analyze the spatiotemporal variations of extreme streamflow from 1911 to 2020. The most detected trends in flow regimes have started in the last six decades and become more severe from 1991 to 2020, which is likely to be dominated by anthropogenic global warming. The results also indicated that alteration of low pulses in most rivers was associated with an increase (decrease) in winter (summer) flows, suggesting the annual minimum flow in summer frequently contradicts natural hydrologic regimes in Arctic rivers. Southern Finland has experienced higher variations in extreme hydrology over the last century. A new low flow regime was detected for southern rivers, characterized by frequent annual minimum flow in summer instead of winter. Moreover, the annual maximum flow before/after spring dictated a new high-flow regime characterized by frequent double peak flows in this region.

Concentration of organic carbon in Finnish catchments and variables involved in its variations

The majority of the carbon worldwide is in soil. In a river catchment, the tight relationship between soil, water and climate makes carbon likely to be eroded and transported from the soil to the rivers. There are multiple variables which can trigger and accelerate the process. In order to assess the importance of the factors involved, and their interactions resulting in the changes in the carbon cycle within catchments, we have studied the catchments of 26 Finnish rivers from 2000 to 2019. These catchments are distributed all over Finland, but we have grouped them into three categories: southern, peatland and northern. We have run a boosted regression tree (BRT) analysis on chemical, physical, climatic and anthropogenic factors to determine their influence on the variations of total organic carbon (TOC) concentration. TOC concentration has decreased in Finland between 2000 and 2019 by 0.91 mg/l, driven principally by forest ditching and % old forest in the catchment. Old forest is especially dominant in the northern catchments with an influence on TOC of 40.5%. In southern and peatland catchments, average precipitation is an important factor to explain the changes in TOC whilst in northern catchments, organic fields have more influence.

Nutrient export from Finnish rivers into the Baltic Sea has not decreased despite water protection measures

To tackle the symptoms of eutrophication in the open Baltic Sea and Finnish coastal waters, Finland has agreed to reduce both total nitrogen (TN) and total phosphorus (TP) inputs. Due to large investments in treatment of municipal and industrial wastewaters, TP loads started to decrease already in the mid-1970s and the respective TN loads in the mid-1990s. During the last two decades, much effort has been spent in decreasing the load originating from diffuse sources. Trend analyses in 1995–2016 showed that, despite various mitigation measures, riverine nutrient export has not substantially decreased, and especially the export from rivers draining agricultural lands remains high. In some areas TN concentrations and export were increasing and we found evidence that it was linked to ditching of peatlands. Several factors connected to climate/weather (e.g. temperature and precipitation) have counteracted the mitigation measures, and therefore Finland will not achieve the nutrient reduction targets by 2021.

PFASs in Finnish Rivers and Fish and the Loading of PFASs to the Baltic Sea

The concentrations of per- and polyfluoroalkyl substances (PFASs) in the Finnish aquatic environment were measured in riverine waters and in inland, coastal and open sea fish. In addition, the PFAS load to the Baltic Sea from 11 rivers was calculated. Measurements show that PFASs, including restricted perfluorooctane sulfonic acid (PFOS), are widely present in the Finnish aquatic environment. At three out of 45 sampling sites, the concentration of PFOS in fish exceeded the environmental quality standard (EQS) of the Water Framework Directive (WFD). The annual average (AA) ∑23PFAS concentration in surface waters ranged from 1.8 to 42 ng L−1 and the concentration of PFOS exceeded the AA-EQS in three out of 13 water bodies. In European perch (Perca fluviatilis) and Baltic herring (Clupea harengus membras), the ∑PFAS concentration ranged from 0.98 to 1 µg kg−1 f.w. (fresh weight) and from 0.2 to 2.4 µg kg−1 f.w., respectively. The highest concentrations in both surface water and fish were found in waters of southern Finland. The riverine export of ∑10PFAS to the Baltic Sea from individual rivers ranged from 0.4 kg yr−1 to 18 kg yr−1. PFAS concentrations in fish of point-source-polluted sites and coastal sites were higher compared to fish of open sea or diffusely polluted sites. The PFAS profiles in surface waters of background sites were different from other sites. This study shows that PFASs are widely found in the Finnish aquatic environment. Different PFAS profiles in samples from background areas and densely populated areas indicate diverse sources of PFASs. Although atmospheric deposition has a substantial influence on PFAS occurrence in remote areas, it is not the dominant source of all PFASs to the aquatic environment of Finland. Rather, wastewaters and presumably contaminated land areas are major sources of PFASs to this aquatic environment.

Establishing minimum counts for semiquantitative bank‐to‐bank river transect mussel studies in species‐poor rivers

AbstractRiverine mussel investigations involve laborious fieldwork, yet acquiring adequate sample sizes is of high importance. This paper presents the calculated minimum sample size required for the bank‐to‐bank transect method in species‐poor rivers. Data were collected from 50 transect dives from four Finnish rivers, and individual‐based rarefaction calculations were used to detect the minimum sample size at which the number of new species plateaus (i.e., species richness curve reaches an asymptote). The results indicate that a minimum of 100 mussels per transect is necessary for general semiquantitative investigations using bank‐to‐bank transect surveys. Because riverine systems typically exhibit high variation in habitats, expert evaluation should always be used to determine the number and location of transects. The effort must be increased if very rare species need to be collected.

Bringing back ecological flows: migratory fish, hydropower and legal maladaptivity in the governance of Finnish rivers

ABSTRACTHistorically, Finnish rivers supported vital populations of migratory salmonids. Presently, these species are more or less endangered due to extensive damming and hydropower production. In this article, we study the main legal and scientific drivers for re-evaluating some of the existing hydropower operations in Finland. We argue that there is a need for re-evaluation on the basis of legal obligations stemming largely from EU law and new scientific knowledge. Theoretically, our setting opens up a classical adaptive governance problem in how to address laws and past decisions that are based on outdated assumptions about the functioning of social-ecological systems.

Long-term trends (1975–2014) in the concentrations and export of carbon from Finnish rivers to the Baltic Sea: organic and inorganic components compared

Finnish rivers exported annually on average 1.2 M t carbon, and total organic carbon (TOC) comprised the major share (nearly 80 %) of this export. The mean area specific carbon export was 4.5 g C m−2 year−1. The highest organic carbon export originated from peat dominated catchments, whereas rivers draining agricultural catchments had the highest area-specific inorganic carbon fluxes. Between 1975 and 2014 total inorganic carbon (TIC) concentrations rose more steadily than the respective TOC concentrations. There was an overall decrease in TOC concentrations between 1975 and the mid-1990s. Decreased point source loading contributed to this pattern, although decreases were also detected in rivers without any major pollution sources. From the mid-1990s TOC concentrations started to rise and the increase was even more pronounced than the earlier decrease. The upward trend was ubiquitous, both in time and space, and it was not possible to link the changes to any specific catchment characteristics or another single driver. Warming climate, changes in hydrology and decreases in acidic deposition were the major driving factors although their contribution varied geographically. At the same time both TOC and TIC export increased slightly, but the strong upward trends in TOC concentrations were not reflected as clearly in TOC export trends. This was because changes in water flow had a dominant influence on TOC export to the sea and any changes in concentrations were masked behind the variation in flow.

Influence of climate and land use changes on nutrient fluxes from Finnish rivers to the Baltic Sea

The ecological state of the Baltic Sea and the inland waters has raised interest in reducing nutrient loads from anthropogenic sources. In this study our main aim was to estimate the trends in agricultural nitrogen and phosphorus fluxes and to explain them by large scale changes in land use and climate. We used long-term monitoring data of discharge and nutrient concentrations of seven small catchments and 20 river basins. In the analysis we used two regression based models to calculate nutrient fluxes and to make source apportionment, and Mann-Kendall statistics to study trends in nutrient concentrations and climatic variables. Annual mean temperature increased at six meteorological stations out of the 16 stations included in the study. There was a statistically significant increase in temperature in late summer/early autumn at 14 stations. There were no trends in annual mean runoff in any of the river basins but there was a decrease in runoff in spring or early summer in 15 and an increase in mid-winter runoff in 13 river basins. There was an increasing trend in the nitrogen concentrations and a decreasing trend in the phosphorus concentrations. The flow normalized nitrogen flux to the Baltic Sea increased steadily until the mid 2000s and has decreased by 20% from that time until 2012. The flow normalized phosphorus flux has steadily decreased from early 1990s to 2012. The specific agricultural nitrogen loading value was 1340kgkm−2yr−1 and the phosphorus loading value 83kgkm−2yr−1. Clearing of new fields explained 50% of the increase in the nitrogen flux to the Baltic Sea between the periods 1995–1999 and 2000–2006. September mean temperature explained 15–30% of the variance in the nitrogen concentrations in the small catchments and 53% of that in the large river basins. Changes in field area or climate did not explain changes in the phosphorus fluxes.

Phosphorus and nitrogen fluxes carried by 21 Finnish agricultural rivers in 1985-2006.

The Finnish Agri-Environmental Programme aims to reduce nutrient load to waters. Using national monitoring data, we estimated the agricultural load (incl. natural background) of total phosphorus (TP) and total nitrogen (TN) transported by 21 Finnish rivers to the northern Baltic Sea and analysed the flow-adjusted trends in the loads and concentrations from 1985 to 2006. We also related the loads to spatial and temporal patterns in catchment and agricultural characteristics. Agricultural load of TN increased, especially in the rivers discharging into the Bothnian Bay, while the load of TP decreased in most of the rivers, except those discharging into the Archipelago Sea. The trends may partly be related to a decrease in grassed area (TP, TN) and increased mineralisation (TN), but the available data on catchment and agricultural characteristics did not fully explain the observed pattern. Our study showed that data arising from relatively infrequent monitoring may prove useful for analysing long-term trend. The mutual correlation among the explaining variables hampered the analysis of the load generating factors.

Spatial and temporal variability of organic C and N concentrations and export from 30 boreal rivers induced by land use and climate

Climate change scenarios for northern boreal regions indicate that there will be increasing temperature and precipitation, and the changes are expected to be larger in winter than in summer. These precipitation and discharge patterns, coupled with shorter ice cover/soil frost periods in the future would be expected to contribute significantly to changing flow paths of organic matter over a range of land use patterns. In order to study the impact of climate change on the seasonality of organic matter export we compared total organic carbon (TOC) and total organic nitrogen (TON) concentrations and export, during different seasons and climatically different years, over 12years for 30 Finnish rivers separated into forest, agriculture and peat dominated catchments. The mean monthly TOC concentrations were highest during autumn and there was also a peak in May during the highest flow period. The mean monthly concentrations of TON were lowest during winter, increased in spring and remaining high throughout summer and autumn. The TOC/TON ratios were lowest during summer and highest during winter, and in all seasons the ratios were lowest in catchments with a high proportion of agricultural land and highest in peat-dominated catchments. The seasonality of TOC and TON exports reflected geographical location, hydrology and land use patterns. Most of the TOC and TON were transported during the high flow following the spring snowmelt and during rainfall in autumn. In all catchments the relative importance of the spring snowmelt decreased in wet and warm years. However, in peat-dominated catchments the proportion of spring period was over 30% of the annual export even in these wet and warm years, while in other catchments the proportion was about 20%. This might be linked to the northern location of the peat-dominated catchments and the permanent snow cover and spring snowmelt, even in warm years.

Estimating aggregated nutrient fluxes in four Finnish rivers via Gaussian state space models

Reliable estimates of the nutrient fluxes carried by rivers from land‐based sources to the sea are needed for efficient abatement of marine eutrophication. Although nutrient concentrations in rivers generally display large temporal variation, sampling and analysis for nutrients, unlike flow measurements, are rarely performed on a daily basis. The infrequent data calls for ways to reliably estimate the nutrient concentrations of the missing days. Here, we use the Gaussian state space models with daily water flow as a predictor variable to predict missing nutrient concentrations for four agriculturally impacted Finnish rivers. Via simulation of Gaussian state space models, we are able to estimate aggregated yearly phosphorus and nitrogen fluxes, and their confidence intervals.The effect of model uncertainty is evaluated through a Monte Carlo experiment, where randomly selected sets of nutrient measurements are removed and then predicted by the remaining values together with re‐estimated parameters. Results show that our model performs well for rivers with long‐term records of flow. Finally, despite the drastic decreases in nutrient loads on the agricultural catchments of the rivers over the last 25 years, we observe no corresponding trends in riverine nutrient fluxes. Copyright © 2013 John Wiley & Sons, Ltd.

Effective size and genetic composition of two exploited, migratory whitefish (Coregonus lavaretus lavaretus) populations

Large scale harvesting and other anthropogenic activities have caused severe population declines in many commercially important fish populations, but accurate information about census and effective population size is often hard to come by. Available evidence suggests that in marine fishes, effective population size (N e) is often several orders of magnitude smaller than census size, such that intensively harvested populations may be particularly vulnerable to loss of genetic diversity. The European whitefish (Coregonus lavaretus) has a long history of heavy exploitation in the Baltic Sea, and the Finnish commercial catch of the species has been substantially reduced, despite high fishing effort. We investigated the temporal genetic stability of migratory whitefish populations from two Finnish rivers (Tornionjoki and Kiiminkijoki), sampled at least twice between 1981 and 2006, by assaying variability in 21 microsatellite loci. Our results suggest a small, albeit significant (F ST = 0.004; p = 0.008) and temporally stable, degree of differentiation between rivers. However, in contrast to earlier reports, heterochronous runs (ascending groups) from Tornionjoki did not exhibit significant genetic divergence. Bayesian estimates of N e suggest substantial declines from historic levels dating to ca 250 years. Yet despite a probable decrease in census population size over the study period, we detected no significant change in contemporary N e. Within group genetic diversity appeared largely unchanged over this time frame; however, we detected a trend towards decreased differentiation between spawning groups (rivers) since the 1980s. These results are discussed in light of stocking programs and conservation of genetic diversity of natural populations.

36 year trends in dissolved organic carbon export from Finnish rivers to the Baltic Sea

Increasing dissolved organic carbon (DOC) concentrations in lakes, rivers and streams in northern mid latitudes have been widely reported during the last two decades, but relatively few studies have dealt with trends in DOC export. We studied the export of DOC from Finnish rivers to the Baltic Sea between 1975 and 2010, and estimated trends in DOC fluxes (both flow normalised and non-normalised). The study encompassed the whole Finnish Baltic Sea catchment area (301,000km2) covering major land use patterns in the boreal zone. Finnish rivers exported annually over 900,000t DOC to the Baltic Sea, and the mean area specific export was 3.5t km−2. The highest export (7.3t km−2) was measured in peat dominated catchments, whereas catchments rich in lakes had the lowest export (2.2t km−2). Inter-annual variation in DOC export was high and controlled mainly by hydrology. There was no overall trend in the annual water flow, although winter flow increased in northern Finland over 36years. Despite the numerous studies showing increases in DOC concentrations in streams and rivers in the northern hemisphere, we could not find any evidence of increases in DOC export to the northern Baltic Sea from Finnish catchments since 1975.

River Lamprey Lampetra fluviatilis (L.) Fishing in the Area around the Baltic Sea

The river lamprey (Lampetra fluviatilis) was previously caught in large numbers in Europe when migrating up in the rivers during autumn for spawning the next spring. It was used as food and was also used as bait in cod fishing in the North Sea. Today the river lamprey has decreased in numbers over much of it’s distribution range, but in the Baltic Sea area, the population is still at a fairly good level, and fishing for lampreys as food (a tradition going back to at least the fifteenth century) is still going on in northern Swedish and Finnish rivers, as well as in coastal rivers in the southern Baltic Sea area. In this article the current situation as regards river lamprey fishing in Sweden, Finland, Latvia and, to some extent, Estonia, Lithuania and Poland is presented.

Clarification of the systematic position ofCercariaeum crassumWesenberg-Lund, 1934 (Digenea), based on karyological analysis and DNA sequences

Chromosome set and rDNA sequences of the larval digenean Cercariaeum crassum were analysed in order to clarify its systematic position and possible adult form. Parasites were obtained from the sphaeriid bivalve Pisidium amnicum, collected in Lithuanian and Finnish rivers. The karyotype is shown to consist of five pairs (2n = 10) of large, up to 14 μm, chromosomes. Complement, composed of a low diploid number of exclusively bi-armed elements, presumably arose through Robertsonian fusions of acrocentric chromosomes. Consistent with a Robertsonian-derived karyotype, one or two small, metacentric, mitotically stable B chromosomes were detected in the cells of parthenitae isolated from some host individuals. A phylogenetic analysis using rDNA internal transcribed spacer 2 (ITS2) and 28S sequences corroborates the allocation of C. crassum to the family Allocreadiidae. In neighbour-joining and maximum parsimony phylogenetic trees C. crassum clusters into one clade with Allocreadium spp., and is the closest sister group in relation to A. isoporum; the level of rDNA sequence divergence between them (2.67% for ITS2 and 1.16% for 28S) is consistent with the level expected for intrageneric variation. The present study adds significant information to a database for establishing species-specific characters for confident characterization of different developmental stages of allocreadiid species, clarification of their life cycles and evaluation of intra- and interspecific variability.

Long-term trends and variation of acidity, COD Mn and colour in coastal rivers of Western Finland in relation to climate and hydrology

High acidity caused by geochemical processes and intensive land use of acid sulphate (AS) soils have continuously degraded the status of water bodies in Western Finland. Despite this, research on the long-term pattern and dynamics of acidification in rivers affected by acid sulphate soils is scarce. This study examined changes in alkalinity and pH value during the period 1913–2007 in nine large Finnish rivers discharging into the Gulf of Bothnia. In addition, patterns of COD Mn and colour were analysed during the period 1961–2007. Relationships between pH, alkalinity, COD Mn and colour and climate variables were also studied. In four rivers with no AS soil impact (Kokemäenjoki, Kemijoki, Iijoki and Oulujoki), critically low pH levels did not occur during the study period, whereas three rivers exposed to minor or moderate levels of runoff from AS soils (Lestijoki, Kalajoki, and Siikajoki) had all periods with critically low pH and alkalinity. The most severe acidity problems occurred in the rivers Kyrönjoki and Lapuanjoki, with extensive drainage of AS soils being the main reason for the low pH status. Maximum discharge was clearly related to the acidity status of many rivers during the autumn–winter runoff period, when a significant negative linear correlation was found between maximum discharge and minimum pH in the rivers affected by AS soils. There was also a more distinct relationship between maximum chemical oxygen demand (COD Mn) and minimum pH in autumn runoff than in spring. COD Mn levels significantly increased with increasing discharge in the rivers with no or minor AS soil impact. Climate change is predicted to increase river flow in general and winter discharge in particular, and therefore the acidity problems in affected rivers may increase in a future climate.

Reviewing the History of Natural Sciences Research on the Bothnian Sea, 1975–2008

The Bothnian Sea is a shallow body of brackish water andpart of the northern-most limb of the Baltic (Fig. 1). Withsalinity averaging 5–6 per mille, a ragged coastline, andslow sedimentation rates (Voipio 1981; Grano¨ et al. 1999),it forms its own unique environment. The A˚ land Islandsand the shape of the seabed prevent the nutrient-richbottom water from Baltic proper from flowing into theBothnian Sea. This constitutes the main reason whyeutrophication has not yet become a major problem here.However, the situation is now beginning to change as aresult of nutrient-rich water draining into the Bothnian Seafrom the many Swedish and Finnish rivers (Lundberg et al.2009). While the Bothnian Sea is an economically impor-tant region with the majority of herring in the Baltic beingcaught here (Parmanne 1998), it has generally not been thefocus of much scientific effort and the reason for this couldbe its rather good condition. The same is true for moni-toring as there are fewer sites, temporally and spatially, inthe Bothnian Sea than in the smaller Archipelago Sea(HELCOM 2009). Consequently it is difficult to conductspatially representative surveys with an insufficient spatialdistribution of sites (Erkkila¨ and Kalliola 2007). Throughreviewing the past 33 years of research on the BothnianSea (1975–2008) (Saikku et al. 2009), we have identifiedgaps in the current knowledge allowing us to providerecommendations for future research focus.The website ISI Web of Knowledge and the ScienceCitation Index Expanded database was searched for articlesrelating to the Bothnian Sea (defined by the latitudes of60.5 N and 63.5 N, and the coasts of Finland and Sweden,Fig. 1). Research work with a focus outside of this region,with no research carried out within the designated BothnianSea area, has not been taken into consideration. The focusof the review is on research into the natural environmentand the human impact on this environment published inEnglish peer-reviewed international and ISI-recognizedscientific journals. This may, therefore, exclude articlespublished particularly in the 1970s and 1980s. A keywordsearch was also performed with ‘‘Gulf of Bothnia’’ as acheck for articles which, while mainly dealing with theGulf, included research carried out in the Bothnian Searegion as well. All articles which came up in the searcheswere manually checked. A final dataset of 132 articlesreferring to the Bothnian Sea were reviewed and summa-rized and these articles were divided under 7 main thematicresearch topics (Fig. 2; Table 1). These research topicswere chosen to describe the natural sciences research donein the Bothnian Sea area, and an article was categorizedinto a research topic based on its major research theme alsogiving potential information on past and future directionsof research.RESEARCH VOLUME AND QUANTITYBothnian Sea research was carried out in small groups ofscientists with the publications most commonly authoredby 1–3 people. A total of 327 different scientists carried outresearch over the past 33 years, but 80% of them publishedonly one article on the Bothnian Sea during this time.Many scientists are, therefore, not continuously researchingor at least not continuously publishing research relating tothe Bothnian Sea. This would mean that the Bothnian Searegion was not a constant and career-spanning topic ofresearch. However, instead of suggesting a lack of interest,

Optically black waters in the northern Baltic Sea

The peculiar bio‐optical properties of the northernmost waters of the Baltic Sea (Bothnian Sea and Bothnian Bay) are illustrated by exploiting unique in situ measurements of apparent and inherent optical properties collected in 2007. Specifically, this study shows that the high values of absorption by colored dissolved organic matter (ays(412) around 1.4 m−1) resulting from the presence of humic matter brought by the numerous Swedish and Finnish rivers alimenting the basin are not correlated with high concentrations of particles as generally observed in the southern Baltic Sea. As a consequence, extremely low values (i.e., less than 0.5) of the ratio of scattering to absorption are observed in the blue spectral region. The corresponding spectra of normalized water leaving radiance are close to satisfy the “black pixel” assumption in the blue and even in the green part of the visible spectrum. Consequently, the Bothnian Sea and most of all the Bothnian Bay, appear ideal candidates for studies on atmospheric corrections and vicarious calibration of ocean color sensors in coastal waters.