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Seasonal variation in size structure and production of autotrophic plankton community in eutrophied, low-light environment: A focus on Mesodinium rubrum

Labucis Atis, Labuce Astra, Jurgensone Iveta, Barda Ieva, Andersone Ingrida, Ikauniece Anda

Oceanologia

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2023

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Vol. 65 (2)

398--409

EN

Temporal variations in the primary production of the size-fractionated autotrophic plankton community were studied in coastal-estuarine waters of the eutrophic Gulf of Riga, Baltic Sea. The community was net-autotrophic during spring and summer and net-heterotrophic during autumn. The results of the present study clearly demonstrate strong covariation between net primary production (NPP) and <56 µm fractionated community biomass, particularly small-sized (16–33 µm) Mesodinium rubrum, implying that the majority of NPP stems from the lower end of the size spectrum. A pronounced size distribution shift was observed within the M. rubrum population. Large-sized (length ≥34 µm) M. rubrum was the most abundant in the first half of the productive season (until week 24), whereas small-sized M. rubrum dominated during the stratified period.

2

Primary productivity in the Gulf of Riga (Baltic Sea) in relation to phytoplankton species and nutrient variability

Purina I., Labucis A., Barda I., Jurgensone I., Aigars J.

Oceanologia

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2018

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No. 60 (4)

544--552

EN

The seasonal patterns of primary production, phytoplankton biomass, chlorophyll a, and nutrients were investigated in the central part of the Gulf of Riga (Baltic Sea) during 2011 and 2012. Annual primary productivity in the gulf was in the range of 353.4–376.2 gC m−2. Maximum carbon fixation rates occurred during the phytoplankton spring bloom from April to May when the winter nutrient pool was rapidly exhausted, suggesting the use of regenerated nutrients already in spring. The new production calculated on the draw-down of nitrates amounted to 51.80% of spring net community production. The production rates during summer were considerably lower owing to the availability of only regenerated nutrients and limited nitrogen fixation. Autumn was established as the least productive season. In autumn despite the elevated nutrient concentrations, the increasingly limited light hindered photosynthetic activity. Species governing the nutrient fluxes and the productivity of the Gulf of Riga are the diatom species responsible for new production in spring. The photosynthetic ciliate Mesodinium rubrum ((Lohmann) Hamburger & Buddenbrock 1911) prevailed in all seasons and significantly correlated with elevated productivity, while diazotrophic cyanobacteria Aphanizomenon flosaquae (Ralfs ex Bornet & Flahault 1886) contributed to new production in the summer nutrient regenerating system.

3

On the buoyant sub-surface salinity maxima in the Gulf of Riga

Liblik T., Skudra M., Lips U.

Oceanologia

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2017

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No. 59 (2)

113--128

EN

Thermohaline structure in the Gulf of Riga (GoR) was investigated by a multi-platform measurement campaign in summer 2015. Stratification of the water column was mainly controlled by the temperature while salinity had only a minor contribution. Buoyant salinity maxima with variable strength were observed in the intermediate layer of the Gulf of Riga. The salinity maxima were likely formed by a simultaneous upwelling–downwelling event at the two opposite sides of the Irbe strait. The inflowing salty water did not reach the deeper (> 35 m) parts of the gulf and, therefore, the near-bottom layer of the gulf remained isolated throughout the summer. Thus, the lateral water exchange regime in the near bottom layer of the Gulf of Riga is more complicated than it was thought previously. We suggest that the occurrence of this type of water exchange resulting in a buoyant inflow and lack of lateral transport into the near-bottom layers might contribute to the rapid seasonal oxygen decline in the Gulf of Riga.

4

Characteristics and inter-annual changes in temperature, salinity and density distribution in the Gulf of Riga

Skudra M., Lips U.

Oceanologia

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2017

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No. 59 (1)

37--48

EN

Available CTD profiles from the Gulf of Riga (May–August, 1993–2012) were analyzed to study inter-annual and long-term changes in temperature, salinity and density in relation to river runoff and atmospheric forcing (e.g. Baltic Sea Index). To describe temporal changes in vertical stratification, the upper mixed layer (UML) and deep layer (DL) parameters were estimated. On average the UML depth increases from 8.7 m in May to 9.0, 11.5 and 13.7 m in June, July and August, respectively, and the UML temperature increases from 8.0°C to 12.5, 18.7 and 18.6°C (May, June, July and August) while the UML salinity increases from 4.90 g kg−1 to 5.14, 5.28 and 5.38 g kg−1, respectively. High correlation (r = −0.82) was found between the inter-annual changes in river runoff (spring) and mean salinity in the UML in August as well as between DL mean salinity (r = 0.88) and density (r = 0.84) in the Irbe Strait and DL mean salinity and density in the Gulf of Riga. Inter-annual changes in the UML depth as well as in DL salinity and density had a significant correlation with the changes in Baltic Sea Index. The strongest stratification (August) can be observed in the years with the highest UML temperature and the highest river run-off in spring. We suggest that the predicted increase in water temperature and changes in river run-off due to the climate change would result in faster development of the seasonal thermocline in spring and stronger vertical stratification in summer.

5

Baltic herring prey selectively on older copepodites of Eurytemora affinis and Limnocalanus macrurus in the Gulf of Riga

Livdāne L., Putnisb I., Rubeneb G., Elfertsb D., Ikauniecea A.

Oceanologia

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2016

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No. 58 (1)

46--53

EN

Zooplankton availability is a major factor affecting herring body condition that in turn describes its well-being. As herring feeding is known to be selective, it is relevant to access its preferences upon zooplankton species and particular copepod developmental stages to forecast possible intraspecific competition for resources in the species scarce environment of the Gulf of Riga where herring stock size due to successful recruitment has almost doubled since 1989. This study tries to answer whether the small-sized plankters dominated zooplankton community permits herring to be a selective eater. Also how herring body condition has changed in connection to environment driven zooplankton community changes. The time series of zooplankton abundance and herring condition from 1995–2012 were studied; and a detailed study of herring diet was performed monthly by stomach content analysis during the main feeding season in 2011 and 2012. We found that herring selectively prey on Limnocalanus macrurus and older copepodite stages of Eurytemora affinis, and moreover these were species of whose selected copepodite stages explained most of variation in herring condition factor. The found relationship between herring feeding selectivity and long-term variation of herring condition allows applying spring zooplankton abundance of E. affinis and L. macrurus to estimate favourable feeding conditions for herring, and could also require the revision of currently used model for herring recruitment estimations, where only biomass of E. affinis is taken into account. In recent years, the high condition of herring can be associated with a considerable increase of lipid-rich copepod species L. macrurus.

6

Streamlined topographical features in and around the Gulf of Riga as evidence of Late Weichselian glacial dynamics

Tsyrulnikov A., Tuuling I., Hang T.

Geological Quarterly

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2008

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Vol. 52, No 1

81-81

EN

Based on various cartographic sources, a digital terrain model and acoustic profiling data, linear relief features of glacial origin have been distin guished and analysed in the Gulf of Riga and adjacent mainland areas in order to reconstruct the dynamics and pathways of former ice streams. North-east–south-west oriented features in the till topography prevail in the central part of the gulf and along the southern coast of the island of Saaremaa, which corroborate the previously known south/south-east di rection of the main Riga ice stream. North-east to south-west directed features dominate in the Pärnu Bay and around the Irbe Strait. Similar deviations from the Riga ice stream are most likely due to ice divide zones, namely the Sakala Upland in Southern Estonia and Kurzeme in north western Latvia, which locally changed the course of the main ice flow. The influence of the Kurzeme ice divide is traceable at the bottom of the gulf up to the southern coast of Saaremaa. There is no evidence of an ice-marginal zone cross ing the central part of the Gulf of Riga as was supposed earlier. The Pandivere-Neva and Palivere ice-marginal zones, which merge on the Sorve Peninsula, probably continue offshore into the Irbe Strait. As the age of the glacier relieffeatures is poorly contained, the chronologi reconstruction of the ice dynamics is tentative.

7

Coherent current oscillations and water exchange in the straits of the Gulf of Riga

Talpsepp L.

Oceanologia

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2005

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No. 47 (2)

115-127

EN

The water exchange processes through the Irbe and Virtsu (Suur) Straits were investigated in 1993-1997 within the framework of a five-year study programme - the Gulf of Riga Project. Simultaneous current measurement data from autonomous mooring stations in both straits were available for the analysis in two periods. In addition to the dominant signals - inertial oscillations in the Virtsu Strait and diurnal oscillations in the Irbe Strait - low-frequency oscillations were found in both straits. During the experiment in July-August 1994, 12-14-day oscillations were observed in both straits: the maximum phase lag in the Virtsu Strait was 1 day. The other important low-frequency periodic component in both straits was 88 hours. In this case, the phase lag in the Virtsu Strait was about 20 hours. In the 1995 experiment in the Irbe Strait, 42-hour oscillations were observed with a phase lag of 10-12 hours. The amplitude was about 30 cm -1 in both straits. As in Lilover et al. (1998), where the flow regime in the Irbe Strait was observed, we can interpret these oscillations as being wind-generated. The present work shows the existence of these disturbances also in the Virtsu Strait. The 88-hour oscillations observed in July-August 1994 can be interpreted as the first mode of the basin's eigenoscillations according to the concept of Otsmann et al. (1997) of a basin with two separate outlets. The lowest frequency oscillation with the period of 12-14 days seemed to propagate to the Gulf of Riga from the Baltic Proper, but the generating force could not be established because there was no noticeable variability between depressions and anticyclones during that period. Based on the current measurements, two types of water exchange through the Irbe strait were established: the outflow over the whole cross-section of the strait, and a bidirectional flow with an inflow near the southern shore and increasing inflow in the near-bottom layers and an outflow in the northern part of the strait.