Dataset description: The dataset contains annually reported data on water flow, input of nutrients and selected hazardous substances to the Baltic Sea from monitored rivers and unmonitored areas within its catchment area. The data, obtained through national monitoring programmes, is reported by all Contracting Parties to the Helsinki Convention to the HELCOM Pollution Load Compilation (PLC-water) database. The data includes transboundary inputs from territories of countries located within the Baltic Sea catchment but not being Contracting parties to the Helsinki Convention, as measured on the borders of downstream countries. The data is quality assured and approved by Contracting Parties to HELCOM in accordance with the HELCOM Guidelines for the annual and periodical compilation and reporting of waterborne pollution inputs to the Baltic Sea (HELCOM PLC-Water Guideline). Dataset content: The dataset contains he following information: : 1. Source name. Name of river catchment or an unmonitored area. 2. Source code. Pollution Load Compilation (PLC) code of monitored river or an unmonitored area. 3. Country code. Code of country in the Baltic Sea drainage area. The following country codes are possible: DK; EE; FI; DE; LV; LT; PL; RU; SE; BY; UA; CZ; SL. 4. Sub-basin code. Code of the Baltic Sea sub-basin used in HELCOM assessment of inputs of nutrients and hazardous substances. The following sub-basin codes are possible: ARC; BAP; BOB; BOS; GUF; GUR; KAT; SOU; WEB. 5. Parameter. Parameters reported by the contracting parties to the Helsinki Convention to assess riverine inputs of nutrients and selected hazardous substances. : AOX; BOD5; BOD7; Cd; COD-Cr; COD-Mn; Cr; Cu; FLOW; Hg; Ni; NNH4; NNO2; NNO23; NNO3; NTOT; Oil; Pb; PPO4; PTOT; SPM; TOC; Zn. (tonnes/year or flow in m3/s). 6. Year of observation. Country codes used in the dataset correspond to: BY - BELARUS, CZ - CZECH REPUBLIC, DK - DENMARK; EE - ESTONIA; FI - FINLAND; DE - GERMANY; LV - LATVIA; LT - LITHUANIA; PL - POLAND; RU - RUSSIA; SK - SLOVAKIA; SE - SWEDEN; UA - UKRAINE. The Baltic Sea sub-basin codes used in the dataset are: ARC - ARCHIPELAGO SEA; BAP - BALTIC PROPER; BOB - BOTHNIAN BAY; BOS - BOTHNIAN SEA; GUF - GULF OF FINLAND; GUR - GULF OF RIGA; KAT - KATTEGAT; SOU - SOUND; WEB - WESTERN BALTIC. Visualization: The dataset can be retrieved from the HELCOM PLC-Water database and visualized utilizing an online user interface. The following filters are available to generate the dataset: 1. Source of input: monitored river, unmonitored area. Multiple selection possible 2. Country: BELARUS, CZECH REPUBLIC, DENMARK; ESTONIA; FINLAND; GERMANY; LATVIA; LITHUANIA; POLAND; RUSSIA; SLOVAKIA; SWEDEN; UKRAINE. Multiple selection possible. 3. Baltic Sea sub-basin: ARCHIPELAGO SEA; BALTIC PROPER; BOTHNIAN BAY; BOTHNIAN SEA; GULF OF FINLAND; GULF OF RIGA; KATTEGAT; SOUND; WESTERN BALTIC. Multiple selection possible. 4. Year (the first and the last years of the time series): 1995-20… 5. Parameter: AOX; BOD5; BOD7; Cd; COD-Cr; COD-Mn; Cr; Cu; FLOW; Hg; Ni; NNH4; NNO2; NNO23; NNO3; NTOT; Oil; Pb; PPO4; PTOT; SPM; TOC; Zn [tonnes/y, flow in m3/s]. Only one parameter can be selected.

These data sets spans multiple oceanographic, chemical, physical, and geomorphological layers. Bathymetric complexity was captured through broad- and fine-scale Bathymetric Position Indices, along with seafloor depth, slope, and ruggedness. Seafloor characteristics included sediment type* and wave exposure estimates. Water column properties were described using measurements of oxygen, salinity, temperature, pH, and nutrient concentrations (nitrate and phosphate) at both bottom and surface layers, each described by mean values and standard deviations. Stratification were characterized by Secchi depth, mixed layer depth, and sea ice concentration. Productivity was approximated by chlorophyll-a concentration. Surface and subsurface variability in wave conditions and satellite-derived sea surface temperature were also collected. Swedish sediment information published with permission from the Swedish Maritime Authority: Decision 23-06009 (12 Sep 2024). To prepare the layers, we applied a procedure designed to iteratively extrapolate missing values in a raster layer by applying a moving window mean filter. It first aligns the input raster to the target grid (Baltic Sea grid at 250 m resolution) using a specified projection method, then repeatedly fills in missing data based on the mean of neighboring cells within a circular window of user-defined size (2 in this case). This iterative process continues until either all missing values are filled or a predefined number of iterations is reached, with early termination if convergence occurs. This procedure ensures that extrapolation respects the spatial extent of the reference grid and provides progress updates during processing. This metadata covers following predictor layers: Bottom Oxygen (Mean) - Mole concentration of dissolved molecular oxygen in sea water at the bottom (µmol/L) Bottom Oxygen (CV) - Coefficient of variation of dissolved molecular oxygen in sea water at the bottom (%) Bottom Salinity (Mean) - Sea water salinity at seafloor (PSU) Bottom Salinity (CV) - Coefficient of variation of sea water salinity at seafloor (%) Bottom Temperature (Mean) - Sea water potential temperature at seafloor (ºC) Bottom Temperature (CV) - Coefficient of variation of sea water potential temperature at seafloor (%) Broad-Scale Bathymetric Position Index - Bathymetric position index, ratio of average depth within 4 km radius to average depth within 20 km radius (m) Chlorophyll-a (Mean) - Mass concentration of chlorophyll a in sea water (mg/m³) Chlorophyll-a (CV) - Coefficient of variation of mass concentration of chlorophyll a in sea water (%) Coarse substrate - Coarse substrate probability of occurrence (%) Depth attenuated exposure at the seafloor - Depth attenuated exposure at the seafloor Distance from the coastline - Distance from the coastline (m) Distance from the nearest river - Distance from the nearest river (m) Fine-Scale Bathymetric Position Index - Bathymetric position index, ratio of average depth within 500 m radius to average depth within 5 km radius (m) Hard substrate - Hard substrate probability of occurrence (%) Mixed Layer Depth (Mean) - Ocean mixed layer thickness defined by sigma theta (m) Mixed Layer Depth (CV) - Coefficient of variation of ocean mixed layer thickness defined by sigma theta (%) Sand substrate - Sand substrate probability of occurrence (%) Sea Ice Concentration (Mean) - Sea ice fraction (%) Sea Ice Concentration (CV) - Coefficient of variation of sea ice fraction (%) Sea surface height (Mean) - Sea surface height above sea level Sea surface height (CV) - Coefficient of variation of sea surface height above sea level (%) Seafloor Depth - Depth in meters below sea surface (m) Seafloor Rugosity within 3 cell radius - Rugosity within 3 cell radius Seafloor Slope - Steepness of the seafloor slope in degrees (Degrees) Secchi Depth (Mean) - Secchi depth of sea water (m) Secchi Depth (CV) - Coefficient of variation of secchi depth of sea water (%) Soft substrate - Soft substrate probability of occurrence (%) Surface Ammonium (Mean) - Mole concentration of ammonium in sea water (µmol/L) Surface Ammonium (CV) - Coefficient of variation of mole concentration of ammonium in sea water (%) Surface Carbon Dioxide (Mean) - Surface partial pressure of carbon dioxide in sea water (µatm) Surface Carbon Dioxide (CV) - Coefficient of variation of surface partial pressure of carbon dioxide in sea water (%) Surface Nitrate (Mean) - Mole concentration of nitrate in sea water (µmol/L) Surface Nitrate (CV) - Coefficient of variation of mole concentration of nitrate in sea water (%) Surface Oxygen (Mean) - Mole concentration of dissolved molecular oxygen in sea water at the surface (µmol/L) Surface Oxygen (CV) - Coefficient of variation of mole concentration of dissolved molecular oxygen in sea water at the surface (%) Surface pH (Mean) - Sea water pH reported on total scale (pH) Surface pH (CV) - Coefficient of variation of sea water pH reported on total scale (%) Surface Phosphate (Mean) - Mole concentration of phosphate in sea water(µmol/L) Surface Phosphate (CV) - Coefficient of variation of mole concentration of phosphate in sea water (%) Surface Primary Productivity (Mean) - Net primary production of biomass expressed as carbon per unit volume in sea water (mg C/m³/day) Surface Primary Productivity (CV) - Coefficient of variation of net primary production of biomass expressed as carbon per unit volume in sea water (%) Surface Salinity (Mean) - Sea water salinity at surface (PSU) Surface Salinity (CV) - Coefficient of variation of sea water salinity at surface (%) Surface Temperature (Mean) - Sea water potential temperature at surface (ºC) Surface Temperature (CV) - Coefficient of variation of sea water potential temperature at surface (%) Terrain Aspect - Aspect, i.e., direction in which the slope is facing out of 360 degrees (N, S, E, W) Wave Height Max (Mean) - Sea surface wave significant height (m) Wave Height Max (CV) - Coefficient of variation of sea surface wave significant height (%) * Swedish sediment information published with permission from the Swedish Maritime Authority: Decision 23-06009 (12 Sep 2024).

This dataset consists of information and status of the main pollution sources within the Baltic Sea catchment area (the Pollution Hot Spots). The HELCOM pollution Hot Spots were idenitified in 1992 by the HELCOM Joint Comprehensive Action Programme (more information is available on www.helcom.fi). The current status of active and cleaned hot spots is available https://helcom.fi/action-areas/industrial-municipal-releases/helcom-hot-spots/.