1277 Assessment methods, the present state of the environment and the assessed environmental impacts

and oxygen saturation 78–105 %. The water has been mildly alkaline at pH 7.0–8.0, which is typical for brackish water. At times, acidic values caused by the river water have been observed in the surface water at observation point PP6.

Electrical conductivity has been found to match that typical of the Bothnian Bay, with fairly little variation. The conductivity has been typically higher in the water layer close to the bottom than in the surface layer. The lower conductivity values sometimes observed at observation point PP6 in spring and fall reflect the influence of the river water. Continuously operating measurements in front of the Hanhikivi headland have revealed several river water pulses also described by model simulations (VitusLab 2012). The average salinity is 3.0 per mil, which is typical for the sea area. The diluting effect of the river water during high discharge in December and the spring floods of April and May typically lowers the salinity to the level of 2 per mil. The impact of river water is strongest in the surface layer.

Water transparency has varied between 0.3 and 6.5 meters. The best values have been observed in the fall. The most significant reason for turbidity in front of the Han-

hikivi headland is the resuspension of bottom sediment caused by waves and currents. The highest turbidity caused by resuspension of sediment were measured in connection with the Tapani and Hannu storms in December 2011. The lowest turbidity values, on the other hand, have been meas- ured under an ice cover. The water has then been generally more turbid than when measured in June. When measured in June, there has been variation from mainly clear to mildly turbid. Brief strong increases of turbidity have been observed at measuring points in winter, connected with a decrease of water temperature. These observations may have been caused by ice crystals, or frazil ice, forming in the water.

Based on color, the water quality has been mainly good or excellent with the exception of the point closest to the mouth of the Pyhäjoki river, where the color values have varied greatly (8–250 mg/l Pt). The high color values have been caused by humus carried by the river water.

The phosphorus content found in the surface water (4–35 µg/l) has been mainly typical of waters with low nutrient content (Forsberg et al. 1980), but figures that indicate mild eutrophication have been observed at points PP1–PP5 in

Figure 7-12. Temperature in relation to water depth at the deepest observation point PP5 near the Hanhikivi headland from 2009 to 2013. Due to the water depth, stratification is more likely and often more intense at observation point PP5 than at other observation points (PP1–PP4 and PP6).

PP5, temperature (°C), June–August

128 7 Assessment methods, the present state of the environment and the assessed environmental impacts

measurements performed in June and August. The surface water at point PP6 has shown periodic increases of phospho- rus content due to river waters. Measurements made in Octo- ber and November have indicated higher overall levels than those measured in the summer. The level of phosphate phos- phorus, which algae can use, has shown considerable variation (<2–28 µg/l). The highest levels by far have been measured at the surface water of point PP4 in June 2011 (20 µg/l) and at point PP6 in August 2012 (28 µg/l). The value measured at point PP6 in particular is due to the impact of river water.

The total nitrogen content in the entire water mass has var- ied between 160 and 940 µg/l. The highest contents have gener- ally been measured in the fall. The highest value (940 µg/l) was measured in the surface water of point PP6, strongly influenced by river water, in August 2012. Plenty of inorganic nitrogen has generally been available to algae. The nitrate-nitrite nitrogen levels in the entire water mass have varied at <5–290 µg/l and the ammonium nitrogen levels at <5–140 µg/l. The average proportion of inorganic nitrogen of the total nitrogen in the surface layer has been 22 % in June and August.

The chlorophyll a content used as an indication of the phytoplankton levels has in the summer season reflected the generally low nutrient content of the sea area (<7 µg/l) (Forsberg et al. 1980). An exceptionally high value has been measured in November 2011 (12.6 µg/l). The weather in the time period in question had been very warm for the season in the whole of Finland, and the climate conditions had clearly been favorable for algae growth. Based on the pro- portions of inorganic nutrients, primary production had been mostly limited by phosphorus.

The general low nutrient content of the sea area has also shown in the low biomass levels of phytoplankton samples. Only the spring maximum for phytoplankton has been strong (2.7 mg/l at most), consisting nearly exclusively of diatoms (Diatomophyceae). The biomass of other groups of algae was very low. The total biomass has been very low at the measure- ments carried out late in the summer. The algae group with the highest biomass was the golden algae (Chrysophyceae). Other groups with relatively high biomasses were cryptomon- ads (Cryptophyceae), diatoms and green algae (Chlorophyceae). A moderate quantity of cyanobacteria (Nostocophyceae) was found at one sampling location. Due to the low salinity of the water, fresh water species have prevailed. (Palomäki 2009)

7.4.1.5 Type of the seabed

The seabed near the Hanhikivi headland has been studied by carrying out seismic refraction surveys and drilling in 2012 (Sito Oy 2012a, Sito Oy 2012b) and by acoustic-seismic sounding of the navigation channel (Rantataro et al. 2012). Researches of contam- inants in sediment have also been carried out in the Hanhikivi sea area in 2009 and 2012. No contaminated sediment was found, and the general levels of contaminants were low.

The waves and stronger currents at the tip of the headland shape the seabed in the area and cause major resuspension of the finest particles of the bottom sediment. Rocks and boul- ders dominate the seabed in the shallow parts (0–5 meters) of the coastal waters at the tip of the headland. Gravel is found in some sheltered depressions. The proportion of gravel increases

when the water depth increases to 5–10 meters. When the water depth exceeds 10 meters, the seabed is covered by fine sand (125–250 µm). Hard clay lenses can be found below the sand at places. Organic sediment can only be found in shel- tered coves near the shore.

The seabed off the Hanhikivi headland mainly consists of coarse soil (sand and gravel) and rock. Clay is found at places. At the outer edge of the zone, the loose soil is slightly finer than around the harbor. Starting from a 200-meter distance from the mouth of the harbor, the layers of soil are mainly composed of sand mixed with silt and silt mixed with sand (Rantataro et al. 2012). The layer of loose soil over the rock bottom is seven meters thick at most. According to the acous- tic-seismic sounding survey (Rantataro et al. 2012) performed in the navigation channel area, the seabed shows signs of ero- sion caused by storms, currents and pack ice. The surface of the seabed is rocky almost over the entire area, and clay sed- iments are only found at close proximity to the shore and at the outer end of the navigation channel. The fine sand found in the area moves with currents.

The rock surface is on level -9–+1 in the harbor area and on level -7–+2 at the cooling water discharge structure area. In the navigation channel, the height of the rock shows some abrupt variation.

The seabed is hard in the designated marine spoil area and the sediment nearly exclusively medium fine sand or coarser elements, such as coarse sand.

7.4.1.6 Aquatic vegetation

The aquatic vegetation in the sea area surrounding the Han- hikivi headland has been studied in a survey of the current state of underwater nature (Ilmarinen et al. 2009) and in a bioindicator survey in 2012 (Leinikki & Syväranta 2012).

The surveys showed that aquatic vegetation in the area is sparse. The shores of the headland are open and flat, and the most sheltered areas with the most diversity can be found in the shallow coves of Takaranta and Kultalanlahti. The marine spoil area is too deep for any aquatic vegetation.

The following habitat types included in the underwa- ter habitat types of the Baltic Sea as listed by the working group on Finnish endangered habitat types (Raunio et al. 2008) were found on the shores of the Hanhikivi head- land and the waters around it: filamentous algal zone of the hydrolittoral (Least Concern, LC), filamentous algal zone of the sublittoral (Near Threatened, NT), Cladophora aegagropila communities (Data Deficient, DD), bottom dominated by submerged macrophytes (Vulnerable, VU) and Charophyte meadows (Endangered, EN).

The Charophyte meadows classified as endangered were found at the planned cooling water discharge site and in the Takaranta area located a few kilometers away from it, among other locations. Based on observations made in 2012, Cha- rophyte meadows are also fairly common in sheltered coves which can be found to the north and south from the Hanhi- kivi headland. The locations are typically shallow lagoons in beaches where the water depth is very low. Charophyte meadows have also been found in the depth of two meters in locations where a mud bottom was covered by sand.

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