Coastal eutrophication phenomena in the eastern Gulf of Finland

Y. I. Gubelit, N. A. Berezina

Zoological Institute, RAS, Universitetskaya emb. 1,

St.-Petersburg 199034, Russia

I. ABSTRACT

Macroalgae blooms and consequent oxygen depletion are temporal and spatial large-scale effects of eutrophication that can influence negatively on benthic fauna. The biomass dynamics of the algae Cladophora glomerata was studied in shallow-water littoral zone of the Neva estuary during May-October of 2003-2005. Also, mean production rate, biomass contribution of drifting C. glomerata and oxygen conditions in habitats were examined during algae decomposition. Two peaks in C. glomerata biomass, in July and in September, were found in all studied years with maximum (300±100 gDWm-2) in September. Primary production of C. glomerata varied from 3.6 to 7.9 contributing around 90 % in total production rate in this zone. During decomposition (since the middle of July to late August) the biomass of drifting C. glomerata exceeded the biomass of the attached algae, contributing 62 % of total algal biomass in studied area. The depletion in oxygen content in water near the bottom was recorded in the shallow-water zone to 20-m distance from shoreline. This phenomenon can influence negatively on invertebrate communities in shallow-water habitats, facilitating their structural shifts. Keywords: filamentous algae, attached and drifting macroalgae, biomass, production, decomposition, oxygen depletion, Cladophora glomerata, Gulf of Finland

II. INTRODUCTION

The proliferation of fast-growing ephemeral macroalgae in shallow coastal zone of seas increasingly observed worldwide, and is generally considered a symptom of coastal eutrophication [1, 2, 3]. In the Baltic Sea ecosystem unfavourable environmental conditions in the littoral zone appear to be the consequence of large filamentous algae “blooms” and storm activity which dramatically affecting the growth of the macrophytes. The great masses of filamentous algae are detached from hard substrates and transported to shallow littoral zone [4, 5, 6]. The drifting algae can form loose-lying algal mats or accumulate on beaches, representing a serious threat to the biodiversity of coastal areas and recreation. In different parts of Baltic Sea these phenomena are characterized by various scale and damage. The eastern Gulf of Finland is known as one of the most eutrophic areas of Baltic Sea. Macrophyte beds in stony littoral of the eastern Gulf of Finland are dominated by a green filamentous alga Cladophora glomerata (L.) Kütz. Our work aims to study seasonal dynamics of C. glomerata biomass in shallow littoral zone of the eastern Gulf of Finland, examine its production rate, and individuate changes in environments during the filamentous algae decomposition.

III. METHODS

Coastal zone of the eastern Gulf of Finland represent stony-sandy littoral, where stony substrates cover from 10 to 100% of bottom area. Extensive shallow-water areas with favorable photic and topic conditions combined with particular weather conditions facilitate proliferation of macrophytes, including fast growing filamentous algae. During iceless period (from May to October) of 2003-2006, phytobenthos dynamics was studied at shallow water littoral within S-Petersburg health resort area (Zelenogorsk, Figure 1). In May water temperatures reached 9−11°C, summer temperatures varied from 15 to 23 in 2003, from 14 to 27 in 2004 and from 16 to 28°C in 2005.

Measurements of water temperature and oxygen content were conducted with the oxygen meter WTWOxi−330. Phytobenthos was sampled by a cylindrical metal frame with sampling area 0.03 м2 in three replicates. All hard substrates in the cylinder were collected. The attached C. glomerata was scraped off hard substrates with a knife and rinsed with fresh water. The drifting detached algae were sampled by the same way and then separated from attached algae in July of 2005 at four points of littoral transect which located at different distance from shoreline (10, 20 (study site), 30 and 40 m) in order to examine biomass ratio of attached and detached algae. The biomass of the algae (air-dried weight) were estimated as arithmetic mean ± SE (standard error) for each date and then re-calculated per 1 m2 of bottom area.

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Diurnal changes of the water oxygen concentration are used for calculation of production rate in C. glomerata. We applied modificated method of daily oxygen changes in dark and light calibrated vessels. Filamentous algae were detached from substrates, rinsed carefully with clean water and weighted. The algae with weight 0.025 – 0.036 g were put in each vessel with volume 0.25 dm3, filled by filtered water from studied site. Field experiments were carried out in the middle of July of 2002, 2005, 2006. Altogether 12 replicates (6 experimental and 6 control vessels) were exposited during 24 h at depth of 0.5 m. After exposition 100-cm3 sample of water from each vessel was analyzed by Winkler’s method. Production rate were calculated per 1 gram of wet weight of C. glomerata with following recalculation the obtained value per 1 gram of carbon (C) using ratio 10:1 [7].

Fig. 1. Map of the eastern Gulf of Finland with indication of study site.

III. RESULTS AND DISCUSSION

In the study site intensive development of C. glomerata were recorded since the middle of May at water temperature 10ºС. For studied period (2003-2005) average biomass of C. glomerata reached 109.3±36 gDWm-2. Two peaks in algae biomass, in July and in September, were found in all studied years (Fig. 2).

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Fig. 2. Seasonal dynamics of average biomass of Cladophora glomerata at study site. The biomass calculated as a mean for each month of 2003-2005 Maximum C. glomerata biomass was recorded in September, when it reached 300±100 gDWm-2 or 1.5 kgWW-2. The highest algae biomass was recorded in 2003, averaging 450 gDWm-2 (Fig. 3). Mean for season of 2003 biomass was 228.5±50 gDWm-2.

Primary production of the attached C. glomerata varied from 3.6 to 7.9 gCm-2 per day (Table I) averaging 5.2±1.2 gCm-2 per day. It reached maximum value in the middle of July 2006. The diurnal P/B-coefficient varied from 0.1 to 0.38 in different years. The macroalgae C. glomerata contributed around 90% in total primary production of this zone (Table I). Difference in P/B between years may be result of different weather and temperature conditions in these periods. The irradiance, temperature and flow rate are considered as main factors influencing the seasonal cycle of C. glomerata [8, 9].

In the middle on July 2005 the area with drifting algae distributed from shoreline to 40-50 m distance from shoreline and to 60-70 cm of depth, decreasing with an increase of the distance (Fig. 4). The biomass of attached algae ranged from 46 to 200 gDWm-2, while biomass of drifting algae varied between 100 and 1000 gDWm-2. The biomass of the drifting algae was 590 ± 64 gDWm-2 in dry weight at study site (20 m distance from shoreline) that almost 3 times exceeded the biomass of the attached C. glomerata (159 ±37 gDWm-2). At near-shore area (10 m distance) the drifting C. glomerata reached 1180 ± 128 gDWm-2, that

was 9 times more than attached C. glomerata biomass. Mean proportion of the drift was 62 ± 14 % of total algal biomass in the zone of 40-m distance from shoreline.

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Fig 3. Dynamics of biomass of Cladophora glomerata at study site in 2003

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Fig 4. Distribution and biomass proportion of drifting filamentous algae Cladophora glomerata in shallow zone of the eastern Gulf of Finland

TABLE I.

Primary production (PP, gCday-1m-2) of phytoplankton and Cladophora glomerata in littoral zone of the Neva estuary at the depth 0.5 m. Data of 2002 and primary production of phytoplankton according to [10].

2002 2005 2006

gCday-1m-2 % gCday-1m-2 % gCday-1m-2 % PP phytoplankton

0.35 9 0.56 11.9 0.54 6.8

PP C. glomerata 3.6 91 4.14 88.1 7.9 93.2

Total PP 3.95 4.70 8.41

Overgrowth of filamentous macroalgae (Ulva spp, Pilayella littoralis, C. glomerata) called “macroalgal blooms”, being a

wide-observed phenomenon for diverse parts of the Baltic Sea [3, 6, 11, 12 ]. In the eastern Gulf of Finland the main part of C. glomerata beds are distributed to depths of 1.5-2 m and did not penetrate below 5 m [10, 13]. Wave activity is one of the most important factors, influencing negatively on growing C. glomerata at the depths 0-1 m. That can result in destabilizing fluctuation of attached filamentous algae biomass in the shallow areas. In contract, in the deeper areas (1.5-2 m) the macrophytes are not

disturbed by these phenomena and their biomass more stable. At the same time, average for season biomasses of attached C. glomerata at 1.5 m and 0.5 m were similar in 2003-2005, reaching 270±30 and 260±30 gDWm-2, respectively [10].

ACKNOWLEDGMENT

The We thank a lot Dr. Sergey M. Golubkov and Dr. Marina I. Orlova for help with organization of sampling and field experimental works, and Dr. Vera N. Nikulina for data on primary production of phytoplankton in littoral zone. The study was supported by grants of Russian Academy of Sciences Program on Bioresources and Biodiversity Conservation, the Russian Foundation for Basic Research (grant # 5577.2006.4), research project from the Scientific Center of Saint-Petersburg

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