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Water transparency can be measured with optical instruments and estimated with satellite sensors, but such measurements have been widely available for only a few decades. Estimates of water transparency using a white disk called a Secchi disk have been made for over a century and can be used to estimate long-term trends. However, historic in situ measurements of the Secchi depth (ZSd) were irregular in space and time and are difficult to interpret in regular time series due to biases introduced by changing locations and the timing of measurements. Satellite data time series, on the other hand, have consistent resolution in both space and time but cover too short a time to resolve climate-scale trends. We normalized historic ZSd measurements in the Baltic Sea with a satellite-derived mean climatology at 5 d temporal and 4 km spatial resolutions and created a merged time series of ZSd for the last century. The mean ZSd in the Baltic Sea from 1927-2020 decreased by 4.2 ± 0.6 m at a rate of 0.045 ± 0.06 m yr-1. Most of the change happened before 1987, and a further decrease was evident primarily in the satellite data during the 1998-2008 period. After 2008, no significant trend in ZSd and or the coefficient of diffuse light attenuation was detected in the Baltic Sea. However, in some sub-basins of the Baltic Sea, the decrease in ZSd continued even after that. The decrease in spectral water transparency in recent decades was highest in the 412 nm band, indicating an increase in the concentration of chromophoric dissolved organic matter.

The South Brazilian Bight (SBB) is a hydrographically dynamic environment with strong seasonality that sustains a diverse planktonic community involved in diverse biogeochemical processes. The inherent optical properties (IOPs; e.g., absorption and scattering coefficients) of optically actives constituents of water (OACs; phytoplankton, non-algal particles–NAP, and colored dissolved organic matter–CDOM) have been widely employed to retrieve information on biogeochemical parameters in the water. In this study conducted in the SBB, a cross-shelf transect was performed for biogeochemistry and hydrographic sampling during a summer expedition. Our research aimed to determine the distribution and amount of the optically active constituents of water based on their spectral signature, in relation to the distribution of water masses in the region. That allows us to get insights into the biogeochemical processes within each water mass and in the boundaries between them. We observed a strong intrusion of South Atlantic Central Water (SACW) over the shelf, mainly driven by the wind action. With that, phytoplankton development was fueled by the input of nutrients, and increased chlorophyll-a (Chl-a) concentrations were observed within the shallowest stations. Colored dissolved organic matter (CDOM) did not follow the distribution of dissolved organic carbon (DOC). Both CDOM and DOC presented high values at the low salinity Coastal Water (CW), as an indication of the continental influence over the shelf. However, CDOM was inversely correlated with salinity and lowest values were observed within Tropical Water (TW), whereas DOC values within TW were as high as within CW, indicating an autochthonous DOM source. Additionally, a deep Chl-a maximum (DCM) was noticed in the boundary between the TW and SACW. Along with the DCM, we observed the production of fresh, non-colored DOM attributed to the microbial community. Finally, our results suggest that CDOM is photodegraded at the surface of CW. This is mainly due to the Ekman transport effect over the region that traps CW at the surface, making it longer exposed to solar radiation.

The effects of ultraviolet radiation (UVR) on the synthesis of mycosporine-like amino acids (MAAs) in sea-ice communities and on the other UV-absorption properties of sea ice were studied in a three-week long in situ experiment in the Gulf of Finland, Baltic Sea in March 2011. The untreated snow-covered ice and two snow-free ice treatments, one exposed to wavelengths > 400 nm (PAR) and the other to full solar spectrum (PAR + UVR), were analysed for MAAs and absorption coefficients of dissolved (aCDOM) and particulate (ap) fractions, the latter being further divided into non-algal (anap) and algal (aph) components. Our results showed that the diatom and dinoflagellate dominated sea-ice algal community responded to UVR down to 25–30 cm depth by increasing their MAA : chlorophyll-a ratio and by extending the composition of MAA pool from shinorine and palythine to porphyra-334 and an unknown compound with absorption peaks at ca. 335 and 360 nm. MAAs were the dominant absorbing components in algae in the top 10 cm of ice, and their contribution to total absorption became even more pronounced under UVR exposure. In addition to MAAs, the high absorption by chromophoric dissolved organic matter (CDOM) and by deposited atmospheric particles provided UV-protection for sea-ice organisms in the exposed ice. Efficient UV-protection will especially be of importance under the predicted future climate conditions with more frequent snow-free conditions.

The pool of riverine dissolved organic matter (DOM) results from integration of complex catchment processes, and links the terrestrial and coastal systems by transporting organic matter from watersheds to estuaries. In this thesis, field samplings and laboratory experiments were combined to assess the spatio-temporal variation in riverine DOM quantity and quality in three Finnish estuaries discharging to the Baltic Sea. Also, the biogeochemical transformation and removal processes influencing the composition of the DOM pool were studied. Large-scale catchment characteristics were linked to the properties of the riverine DOM. Throughout the work the DOM quality was assessed using multiple analytical approaches: C/N stoichiometry, colored DOM (CDOM) absorption, CDOM fluorescence, molecular weight and iron content. Estuarine DOM was subjected to heterotrophic degradation in factorial experiments to quantify the role of salinity, inorganic nutrients and predegradation to DOM bioavailability and the performance of the heterotrophic bacteria. Additionally salt-induced flocculation of DOM was studied by combining field samplings, laboratory experiments and modeling. The selected three study catchments differed markedly in their land-use, and these differences were reflected on the riverine DOM quantity and quality. Seasonal variation was observed in both the quantity and quality of the riverine DOM, but this variation was not reflected to the degradation processes in the estuaries. The experiments provided evidence that increasing proportion of forests and peatlands were linked to the increase of carbon loading from the catchment, and to decreases in the subsequent quantities of bioavailable dissolved organic carbon (DOC) and bacterial growth efficiencies (BGE). A higher proportion of agricultural land in the catchment indicated an increase of the amount and bioavailability of dissolved organic nitrogen (DON) in the DOM pool. A larger proportion of lakes in the catchments, on the other hand, were related to decreased bioavailable DON. The moderate changes in salinity introduced in the experiments did not affect either DOM bioavailability or BGE. Also replete inorganic nutrients did not influence the DOM bioavailability, although did increase BGE on average from 11 to 40%. Increasing predegradation, i.e. DOM subjected to heterotrophic degradation for varying times before the actual bioassays, decreased BGE from 65 to 25% on average. Flocculation caused deviations from conservative mixing of DOM variables in the study estuaries, and the quantity and quality of flocculated DOM was studied in a laboratory experiment. The maximum deviation from conservative mixing of DOC in estuaries was -16% at salinities between 1 and 2, indicating significant flocculation within a relatively narrow salinity range. Both processes, biodegradation and flocculation, removed riverine DOM before reaching the open sea (so-called marginal filter), but also changed the properties of the remaining DOM pool. Also, both processes increased the humic-like fluorescence and DOC-specific absorbance of the DOM pool, which suggests that the refractory DOM pool reaching the sea is a result of multiple, interacting processes along the hydrological path. All in all, both biodegradation and flocculation remove riverine DOM in estuaries, and also transform the remaining DOM pool that ultimately reaches the open sea. Findings from this thesis show that DOM quality has a pivotal role in the functioning of both of these essential and ubiquitous mechanisms. Jokien kuljettamalla liuenneella orgaanisella aineksella (dissolved organic matter, DOM) on merkittäviä vaikutuksia rannikoiden vesiekosysteemeihin; DOM varjostaa kasviplanktonia ja muuta vesikesvillisuutta sekä kuljettaa suuria määriä hiiltä, typpeä ja fosforia mikrobiravintoverkon käyttöön. Jokiperäinen DOM on tulosta valuma-alueen monimutkaisten prosessien yhteisvaikutuksista, yhdistäen maa- ja rannikkosysteemit kuljettamalla eloperäistä ainesta valuma-alueelta jokisuistoihin. Tässä työssä on yhdistetty kenttähavaintoja ja laboratoriokokeita jokiperäisen DOM:in määrän ja laadun vaihtelun arvioimiseen kolmessa Itämereen laskevan joen suistossa. Lisäksi työssä tutkittiin DOM:in koostumukseen vaikuttavia biogeokemiallisia muutos- ja poistoprosesseja. Valuma-alueen ominaisuuksia pystyttiin yhdistämään jokiperäisen DOM:in koostumukseen. DOM:in laatua arvioitiin tutkimuksessa useilla analyyseillä: C/N stoikiometrialla, värillisen eloperäisen aineen (colored dissolved organic matter, CDOM) absorptiolla ja fluoresenssilla, molekyylipainolla ja rautapitoisuudella. Jokisuistojen DOM altistettiin heterotrofiselle bakteerihajotukselle faktorityyppisissä koejärjestelyissä saliniteetin, epäorgaanisten ravinteiden ja esihajotuksen vaikutusten DOM:in biohajoavuuteen ja bakteeriyhteisön toiminnan tutkimiseksi. Lisäksi suolan aiheuttamaa DOM:in sakkautumista tutkittiin yhdistämällä kenttähavaintoja, laboratoriokokeita ja mallinnusta. Kolme tutkittavana ollutta valuma-aluetta erosivat merkittävästi maankäytöltään, ja nämä muutokset heijastuivat jokiperäisen DOM:in määrään ja laatuun. Vuodenaikaisvaihtelua havaittiin sekä DOM:in määrässä että laadussa, mutta vaihtelu ei vaikuttanut DOM:in biologiseen hajotukseen. Laboratoriokokeet vahvistivat metsä- ja suopinta-alan vaikuttavan lisäävästi valuma-alueiden hiilivirtoihin, mutta vähentävän biohajoavan liuenneen eloperäisen hiilen (dissolved organic carbon, DOC) suhteellista osuutta ja heikentävän bakteerikasvutehoa (bacterial growth efficiency, BGE). Maatalousmaan suuri suhteellinen osuus valuma-alueella voitiin yhdistää korkeaan liuenneen eloperäisen typen (dissolved organic nitrogen, DON) kuormaan ja biohajoavuuteen. Järvien suhteellisesti suuri määrä valuma-alueella taas alensi DON:in biohajoavuutta. Laboratoriokokeissa suolapitoisuuden ei havaittu vaikuttavan DOM:in biohajoavuuteen eikä BGE:hen. Myöskään epäorgaanisten ravinteiden lisäys ei kasvattanut biohajoavuutta, mutta nosti bakteerikasvutehoa keskimäärin 11 prosentista 40 prosenttiin. Esihajotus, eli DOM:in altistaminen vaihtelevalle jaksolle heterotrofista bakteerihajotusta ennen varsinaisia hajotuskokeita, laski kasvutehoa keskimäärin 65 prosentista 25 prosenttiin. Sakkautuminen aiheutti poikkeamia DOM:in konservatiivisesta sekoittumisesta tutkituissa jokisuistoissa, ja sakkautuvan DOM:in määrää ja laatua tutkittiin laboratoriokokeessa. Suurin havaittu poikkeama DOC-pitoisuudessa konservatiivisen sekoittumisen odotusarvosta oli -16% suolapitoisuuden ollessa välillä 1 - 2, joka kertoo merkittävästä sakkautumisesta suhteellisen kapealla suolapitoisuuden vaihteluvälillä. Molemmat prosessit, biohajotus ja sakkautuminen, poistivat jokiperäistä DOM:ia ennen sen kulkeutumista merelle, mutta myös muuttivat jäljelle jääneen DOM:in ominaisuuksia. Molemmat prosessit myös lisäsivät DOM:in humustyyppistä fluoresenssia ja ominaisabsorbanssia, mistä voidaan päätellä että merelle saakka päätyvä, vaikeasti hajotettava DOM on tulosta monista vuorovaikutteisista prosesseista matkalla maalta merelle. Kaiken kaikkiaan sekä biohajotus että sakkautuminen poistavat DOM:ia jokisuistoissa, mutta myös muuttavat jäljelle jäänyttä DOM:ia. Tämän tutkimuksen tulokset osoittavat että DOM:in laadulla on suurta merkitystä näiden keskeisten ja yleisten mekanismien toimintaan.