Eutrophication Index (trix) – An Operational Tool For The Black Sea Coastal Water Ecological Quality Assessment And Monitoring
Eutrophication Index (TRIX) – an operational tool for the Black Sea coastal water ecological quality assessment and monitoring
S. Moncheva, V. Doncheva
Institute of Oceanology, BAS, Varna, Bulgaria
Introduction.
During the last three decades anthropogenic eutrophication has been identified as a key ecological problem, imposing dramatic alterations in the chemical and biological regimes, resulted in destroyed carrying capacity and deteriorated environmental health of the coastal Black Sea ecosystem (in Oszoy, Mykaelian, 1997; Zeitzev, Mamaiev 1997; Mee, Topping, 1999).
In contrast to the economics where well developed indicators give managers powerful tools for decision-making, there are only very week instruments to assess the quality of the marine environment (Hammond et al, 1995). Similar to the field of human health protection, where well-developed indicators and regulations have been successfully employed, the assessment and protection of environmental health requires an adequate system for monitoring, diagnoses and management. Despite the numerous investigations this field of marine research is poorly exploited. GESAMP (1995) has published a list of biological indicators, but they are more applicable to long-term series for measuring the biological response to the evolution of anthropogenic eutrophication.
Hence eutrophication in marine coastal areas has aquired the degree of global importance, terms such as ’oligotrophy’, ’mesotrophy’ and ‘eutrophy’, are more frequently encountered in the marine literature. Accordingly the interpretation of available data is biased by the impossibility to ignore the subjectivism of various scientists, the lack of precision in determining ‘how productive’ the examined waters are and where the boundaries between the trophic categories have to be set (Vollenweider et al., 1998 and references in).
Thus an adequate approach to process trophic data in a unified form is crucial to consent various trophic conditions and to meet the necessity of introducing more indicators of Drivers, Pressure and State (according to the
DPSIR concept, currently in use by the European Environment Agency). Recently a new trophic state index (TRIX) was proposed for classification of marine coastal waters with respect to pelagic trophic state (Vollenweider et al, 1998) to replace OECD (1982) methodology which has mainly been used for freshwater but also applied to marine environment (Giovanardi and Tromellini, 1992).
The objective of the present paper is to test applicability of TRIX for scaling eutrophication and assessment ecological quality in the Bulgarian Black Sea coastal zone with potential implication for monitoring.
Materials and methods.
The selection of the sampling coastal areas (3 miles zone) was made in reference to the antropogenic impact - (Fig.1).
Fig. 1. Map of sampling stations
Cape Kaliakra site is mainly influenced by the Danube river runoff, the Varna Lake-Varna Bay current plume exerts a strong contamination effect on C.Galata site. Shkorpilovtsi site is indirectly influenced by the Kamchia River
outflow. Varna Bay is one of the “hot spot” along the Bulgarian Black Sea coast as subjected to various anthropogenic activities and emissions.
The analysis is based on bimonthly samples (EROS’2000-EROS’21 monitoring Program, NATO SfP Project and NSF Project) during the period 1995-1998. The samples for chlorophyll a and nutrients are analyzed by routine methods, described in detail elsewhere (Shtereva et al., 1999).
The Trophic State Index (TRIX) is defined by the linear combination of four fundamental parameters of surface water quality (Volenweider et al., 1998):
TRIX = Log([Chl]*[D%O]*[PT]*[DIN] +1.5)/1.2;
where: Chl = chlorophyll ‘a’ in µg/L;
DO% = deviation, in absolute value, of Dissolved Oxygen from 100% saturation;
PT = Total Phosphorous in µg/L;
DIN = Dissolved Inorganic Nitrogen in µg/L.
In this study Inorganic phosphorous (IP) was used instead of PT and Si was included as an additional parameter in the formula (after necessary transformations) to test its importance in the variation of TRIX (chemical data are published in EROS’2000/EROS’21 Technical Reports 1997/1999).
Numerically TRIX is scaled from 0-10 covering a wide range of trophic conditions and defining 4 trophic categories (water quality status - WQS): 6 – very high trophic level (poor WQS). A high value (> 6 TRIX units) corresponds to very high nutrient levels, low transparency and recurrent hypoxia/anoxia in bottom waters; while low TRIX values (TRIX
Results and discussion.
The selected sights manifest a high productive potential (high chl. a values, exceeding 10 ug/l and nutrients), insignificant differences in temperature and salinity and as expected high variability in all environmental parameters –Table 1.
Table 1. Statistical summaries of environmental parameters (surface)
Region
T°C
S
DO[ml/l]
P(PO4)[µM]
Nmin[µM]
Si(OH)4[µM]
Chl.a [µg/l]
C.Kaliakra
(1995-96)
Average
13.23
16.13
6.85
0.25
17.23
5.39
7.30
stdev
7.61
0.91
1.46
0.14
15.19
4.61
12.74
min
1.80
14.42
4.85
0.05
2.77
0.00
0.75
max
24.80
17.67
9.28
0.55
52.09
15.26
55.10
median
12.23
16.41
6.21
0.23
9.68
5.53
3.52
C.Kaliakra
(1997-98)
Average
14.15
15.99
6.81
1.62
23.08
5.78
2.16
stdev
5.16
1.10
1.13
4.39
56.58
4.91
1.33
min
4.70
12.17
4.31
0.00
0.75
0.64
0.46
max
22.56
16.83
8.34
18.92
281.32
21.54
4.74
median
13.34
16.44
7.15
0.13
6.40
4.42
2.02
C.Galata
(1995-96)
Average
15.77
16.23
7.27
0.22
24.93
4.93
stdev
8.52
1..32
1.66
0.17
29.27
5.86
min
2.00
13.23
5.74
0.04
2.10
0.75
max
26.20
17.73
11.62
0.68
87.96
24.55
median
15.60
16.73
6.63
0.16
6.70
3.50
Shkorpilovtzi
(1995-96)
Average
13.99
15.82
6.99
0.34
21.57
6.43
3.04
stdev
8.65
1.46
1.35
0.24
25.99
4.57
3.36
min
1.00
12.79
5.17
0.02
1.29
0.00
0.44
max
25.19
17.75
9.06
0.85
89.12
14.74
12.43
median
16.20
16.18
6.90
0.26
8.79
4.96
2.19
Adriatic Sea
(Vollenweider
et al., 1998)
Average
18.00
32.50
5.60
300.50
11.30
stdev
6.90
2.90
4.80
497.90
21.90
min
3.80
16.20
0.10
6.10
0.40
max
27.10
37.40
34.00
4776.00
194.10
median
16.20
32.40
2.90
174.10
5.40
It is worth notice that all the component parameters presented in the table are much higher in the Adriatic Sea (coastal area subjected to the influence of Po river flow) as compared to that off the Black Sea coastal area, which has some bearings in applicability of TRIX as a common and unified measure of trophic conditions.
Fig. 2. Temporal variation of TRIX: TRIX-1 (Si included); TRIX-2 (normal calculations); [Chl.*DO%] - module of actual productivity (in %) from TRIX units.
As apparent from Fig.2. the temporal scale of variation (year cycle) of TRIX for the three sites is also very high but within almost similar range: 3.68-6.81 – C. Kalikra-95/96; 3.71- 7.76 – C. Galata; 2.83 – 6.79 – Skorpilovtzi, for 1997/98 the amplitude of TRIX variability at C. Kaliakra is slightly lower (2.51 – 6.02) as compared to the 95-96 period. The highs normally were measured in winter - spring or autumn and the lows typically in summer months. These stand for both TRIX-1 and TRIX-2 fluctuating in parallel, with slight discrepancy, more evident at C. Kaliakra site (97/98). The analysis of histogram distribution (not shown) however reveal that for the three areas the rate frequency of TRIX units >5 exceeds well 50% out of the total number of sampling cases (Kaliakra 76-54%; Galata - 92%; Shkorpilovtzi – 55%) and that > 6 over 30% (CGalata – 43% and CKaliakra 95/96 – 53%). The seasonal means of TRIX manifest relatively uniform trend of variation in the three sites (highest in winter - spring and lowest in summer) – Fig. 3.
In Varna Bay only (a well recognized highly eutrophicated area) the summer average exceeds 6, in contrast to the 3-mile zone where the summer average is within the range 4-4.5 units.
In average the [Chl.*DO% ] module contributes to more than 50% to the fluctuation of TRIX (Fig.2), chlorophyll a itself accounting for about 30 % of its seasonal variations, which altogether suggests the key role played by phytoplankton for the ecological health of the coastal area.
By annual average the examined sites could be ranked in the following order: Varna Bay (7.35), CGalata (5.18), CKaliakra (5.07) and Shkorpilovtzi – 4.96. According to the proposed scale the three mile zone off the Bulgarian Black Sea coast can be classified as of high trophic level (meso-/eutrophic) and Varna Bay asserted as “eutrophic risk” area.
Fig.3. Seasonal and annual means of TRIX units by sites and areas (data for Adriatic and Aegean Sea are after Vollenweider et al., 1998; Moncheva et al., 1999).
This is quite in conformity with the conclusions derived from analysis of different abiotic and biotic parameters (Moncheva et al, 1995, Shtereva et al. 1999, Velikova et al., 1999), suggesting the ecological relevance of TRIX to scaling of eutrophic conditions. The latter is well supported by the high negative linear correlation (not shown) between TRIX/Shannon-Weaver diversity index by phytoplankton biomass (R2 = 0.63) as a relative measure of communities ecological status.
Obviously the coastal areas of the Aegean, Adriatic and Black Sea manifest similar rate of eutrophication (if Varna Bay is excluded the average for the 3 mile zone in Black Sea is 5.1) irrespective of the numerical differences in the parameters (Moncheva et al., 1999).
The general philosophy of an explicit trophic index is based on the following principles: component parameters of the index should be meaningful in terms of both, primary production and productivity dynamics; encompass major causal factors; being a routine measurement in most marine surveys (Vollenweider et al., 1998). Originally the design of TRIX satisfies all these principles. As an integral index TRIX varies within the rate of actual productivity of the ecosystem (Chl.a*DO%) as well as within the range of its potential productivity (nutrients -(N*P), the interaction between these two main modules expressed in relative units. This conforms to the general appreciation that the numerical values of neither nutrients nor phytoplankton biomass could be a relevant expression of eutrophication. What counts is the degree of deviation from the boundary conditions that define the “normal state” of an ecosystem, which alternatively is ecosystem specific.
Presumably no index is a perfect substitute of the original data e.g. the properties and the operational mechanisms of the system that the index represents. The more complex a system, the less it can be described by a few key parameters. Nonetheless the relative scale covering a wide range of trophic conditions, the possibility to set critical levels, together with the composite parameters are the main arguments in favour of TRIX as an unified driver-state type indicator of ecosystem health assessment. Not ignoring the importance of other abiotic (temperature, salinity, light intensity, hydrodynamic) and biotic parameters (graizing pressure, trophic interactions etc.) of the water domain which also could contribute to the manifestation of eutrophic conditions. (Moncheva et al., 2000 ).
Conclusions.
The forgoing analysis provides objectives to recommend the trophic state index TRIX as a relevant tool to:
§ narrow down and score the number of indicators for the evaluation of trophic status and surface water quality and set relevant monitoring programs, providing that the time-scale of sampling is properly defined (already included in the Eutrophication Monitoring Black Sea Programm, Draft, Istanbul, 2000);
§ conciliate considerations on spatial and temporal variability of trophic state over a wide range of environmental conditions;
§ set uniform bases for classification of marine coastal waters according to their trophic characteristics and critical levels for “eutrophic/ecological risk” - it has recently been adopted by the Italian Environmental Legislation (Dlg. 152/99 - Eutrophication Monitoring Black Sea Program, Draft, Istanbul, 2000);
§ provide Local Authorities with easy-to-manage information in decision-making and promoting of remedial actions.
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