Littoral zone of the Gulf of Riga by the Engure lake – Mērsrags transect

1997 – up to now (every second year).

Elmīra Boikova.

• The first one (Mersrags habitat) was located on the west coast of the Gulf, in the Engure Nature protected area. The monitoring activities were based on transects which represent an open area of very similar substrate type. These transects are located on the tip of Mersrags, 2.5 km northwards from the port of Mersrags.
• The second one (Saulkrasti habitat) were placed on the east coast of the Gulf of Riga, there the influence of three rivers inflow is remarkable. This area is with the highest proportion of the hard substrate. The transects were located northwards from the main beach of Saulkrasti.

The quality and biodiversity of the underwater meadows in the littoral zone of the Engure lake cathcement area.

Longterm studies of the Gulf of Riga coastal zone communities and macrophytes as the key component of littoral zone under natural and anthropogenic impact and possible threats to the littoral zone biological resources.

Following core variables were measured by using SCUBA diving:

• Site position;
• Transect depth profile;
• Substrate
;
• Depth distribution of important plant species;
• Composition of plant species;
• Coverage % of plant species;
• Temperature;
• Water transparency;
• Salinity;
• Nutrients.

Supporting studies at the same time were carried out to get information about the structure of benthos and plankton communities - phytoplankton, zooplankton, microbial food web at the macrophyte investigation transects. On a long term basis heavy metal concentration in sediments and key macrophytes were observed.

The macrophyte monitoring basically is performed accordingly to the “Guidelines for monitoring of phytobenthic plant and animal communities in the Baltic Sea” described in Annex for HELCOM COMBINE programme (26 March 1999). Because there is no obligatory macrophyte communities monitoring programme covering all Baltic states and Water Framework Directive is under development for marine coastal waters, we suggested that this monitoring is attributed to the investigative monitoring.

In macrophyte monitoring areas the algae communities are described on a transect from the water edge to the maximum depths of occurrence of phytobenthos or last shoots.

For all transects along the phytobenthic belt zones were divided into sections accordingly to certain depth interval (1-2m). The samples of biomass were taken for each algae belt, at least three frames in each belt. The frames in size of 0.2 x 0.2 m for the green algae and 0.5 x 0.5 m for the brown and red algae were taken.

SCUBA diving technique was used from depth deeper 1.5m. The depths profiles of transects were recorded by using echo sounder.

During field work preliminary list of species with depth ranges were recorded. All plants and animals within each frame were collected in a plastic bag and transported to the laboratory for deep freezing.

Quantitative samples in the laboratory were sorted to species for plants and animals separately .Each species was counted and dried on aluminum foil. After drying for 48 hours at 60 oC it was constant weight of algae. Samples were weight and the dry weight/m2 was calculated.

Comparison of two marine sites of the Riga Gulf:

• Mersrags – west coast of the Riga Gulf, Lake Engure catchment area;
• Saulkrasti – east coast of the Riga gulf with eutrophication impact from the rivers.

According to long term data the maximum vertical depth of distribution of macrophyte communities is decreasing mainly impacted by eutrophication effect

At the west coast site – Mērsrags, located in the littoral zone bordering drainage basin of Lake Engures there are rich and diverse underwater meadows – about 20 species of macrophytes have been identified and their vertical depth distribution is about 9-10 meters

At the east coast site – Saulkrasti with strong river impact vertical depth distribution of macrophytes is only 4-5 meters, the longterm data illustrates that species number is remarkably lower – only 12. The key species – Fucus vesiculosus is hardly overgrown by epiphytes.