The main scope of the Marine Strategy Framework Directive is to achieve good environmental status (GES) of the EU's marine waters by 2020, in order to protect the marine environment more effectively. The trophic index (TRIX) was developed by Vollenweider in 1998 for the coastal area of Emilia-Romagna (northern Adriatic Sea) and was used by the Italian legislation to characterize the trophic state of coastal waters.
We compared the TRIX index calculated from in situ data (“in situ TRIX”) with the corresponding index simulated with a coupled physics and biogeochemical numerical model (“model TRIX”) implemented in the overall Adriatic Sea. The comparison between in situ and simulated data was carried out for a data time series on the Emilia-Romagna coastal strip. This study shows the compatibility of the model with the in situ TRIX and the importance of the length of the time series in order to get robust index estimates. The model TRIX is finally calculated for the whole Adriatic Sea, showing trophic index differences across the Adriatic coastal areas.
Marine habitats are subject to increasing pressures (as nutrient discharges, eutrophication) due to agriculture, industry, tourism, fishing, and aquaculture. The eutrophication of coastal waters is considered to be one of the greatest threats to the health of marine ecosystems. It is described as a change in the marine food web connected to the seawater enrichment by nutrients, which can modify the carbon pathways and excessive oxygen consumption (Ferreira et al., 2011; Vollenweider et al., 1992).
In response to these pressures, the Marine Strategy Framework Directive (MSFD, 2008/56/EC) explicitly considers eutrophication descriptors as key to determining the good environmental status (GES) of European coastal waters. The EU MSFD addresses the overall state of the marine environment utilizing a DPSIR (driver, pressure, state, impact, response) conceptual approach and considering eutrophication as an important process that can alter the coastal waters' GES. A synthetic indicator of the environmental state of the coastal ocean with respect to the eutrophication processes, integrating elements of the DPSIR methodology, is therefore very useful to provide an objective assessment of the environmental state. Furthermore, it provides elements for the implementation of an ecosystem-based strategy for the achievement and maintenance of GES. The MSFD underlines the need to implement an ecosystem-based approach to determine all the pressures affecting the marine environment relative to GES. Indicators therefore need to be developed to qualitatively and quantitatively assess the quality of the marine environment. Marine ecosystems present high levels of complexity; hence composite indicators are needed to support monitoring programs and reduce complexity for early-warning systems.
Eutrophication assessment indicators should use multivariate water column
state variables, integrating physical-chemical and biological variables.
The trophic index (TRIX) is a eutrophication index proposed by Vollenweider et al. (1998) in
order to characterize the trophic state of marine waters along the
Emilia-Romagna coastal region (northwestern Adriatic Sea). TRIX is defined
by four state variables, which are strongly correlated with primary
production: chlorophyll
TRIX covers a wide range of trophic conditions from oligotrophy to eutrophy, and it has been applied to coastal marine waters in several European seas: the Adriatic Sea and the Tyrrhenian Sea (Giovanardi and Vollenweider, 2004), the Black Sea (Kovalova and Medinets, 2012; Baytut, 2010; Dyatlov et al., 2010; Medinets et al., 2010; Moncheva and Doncheva, 2000; Moncheva et al., 2002; Zaika, 2003), the eastern Mediterranean Sea (Tugrul et al., 2011), the Aegean Sea (Yucel-Gier et al., 2011), the Marmara Sea (Balkis et al., 2012), the Caspian Sea (Shahrban and Etemad-Shahidi, 2010), the Mar Menor (Salas et al., 2008), the Persian Gulf (Zoriasatein et al., 2013), and the Gulf of Finland (Vaschetta et al., 2008).
In this paper we compare “in situ TRIX” with model simulations for long data series in different coastal and open-ocean areas. The specific objectives of our work are (1) to adapt the TRIX generic relation to numerical ecosystem model simulation data, (2) to validate the “model TRIX” with in situ data in different areas and time series, and (3) to apply the TRIX generic equation to other coastal and open-ocean areas in the entire Adriatic Sea.
The final results of this paper could be used as a criterion to classify the marine ecosystem (D.L. 260/2010), providing class boundaries expressed as TRIX units (Table 4.3.2./c). Furthermore, it is shown that the ecosystem simulations can represent an important support for monitoring activities, allowing TRIX to be extended to larger areas where in situ sampling activities are difficult to implement.
Section 2 describes the TRIX equation and its calibration parameters for the model simulations. Section 3 illustrates the in situ and simulation model data used for the evaluation of TRIX and its calibration. Section 4 compares the in situ TRIX and model TRIX, and the sensitivity analysis of the calibration parameters. Section 5 shows how TRIX could be implemented for the whole Adriatic Sea region, and Sect. 6 presents the discussion and conclusions.
The TRIX index was developed by Vollenweider et al. (1992) using data collected between 1982 and 1993 by the “Daphne” oceanographic division of the Emilia-Romagna Regional Environmental Protection Agency (hereafter referred to as ARPAE-Daphne). Since 1971 ARPAE-Daphne has been carrying out a monitoring program (Regione Emilia-Romagna, 1981–2013) covering the whole of the Emilia-Romagna coastal region. The location of the sampling stations is reported in Fig. 1.
Emilia-Romagna coastal and shelf region monitored by ARPAE-Daphne: 21 stations, organized along eight transects (Lido Volano, Porto Garibaldi, Casalborsetti, Ravenna, Lido Adriano, Cesenatico, Rimini, and Cattolica) from 500 m to 10 km distance from the coast. The Porto Garibaldi and Cesenatico transects enclose also two stations situated 20 km offshore. The study area is divided into three areas (A, B, and C) based on hydrological and trophic conditions. The grey shaded areas indicate the model grid points from which model data were extracted to carry out the model TRIX.
Reference values of the trophic index (TRIX) and corresponding water quality and trophic conditions, developed from ARPAE-Daphne Emilia-Romagna (Rinaldi and Giovanardi, 2011).
The TRIX index is based on four state variables (
Vollenweider et al. (1998) further simplified the TRIX formula by assuming
(on the basis of the data used) that the difference (
Equation (2) gives the TRIX index currently used by ARPAE-Daphne and adopted by the Italian national legislation (D.L. 260/2010). For the Italian coastal waters, TRIX values range from 0 to 10: 0 corresponds to extreme oligotrophic conditions; while10 corresponds to extreme eutrophic conditions. TRIX values have been further aggregated into four trophic regimes (Rinaldi and Giovanardi, 2011): “elevated”, “good”, “mediocre”, and “bad” (Table 1). Referring to Italian waters, TRIX values exceeding 6 are typical of highly productive coastal areas, characterized by frequent episodes of sea bottom anoxia (Giovanardi and Vollenweider, 2004).
In the following sections we calculate the TRIX index scaling parameters
In addition to TRIX, an efficiency coefficient can also be defined
(Giovanardi and Vollenweider, 2004) as the logarithm of the ratio between the
two aggregated main components of the TRIX, i.e.,
The efficiency coefficient can be considered a supplementary index with which to evaluate the nutrient utilization of the system. In the present study we also calculate the efficiency coefficient from both in situ data and model simulations.
The in situ data used in this paper were collected by the ARPAE-Daphne monitoring program. We considered the 1982–1993 data time series originally used by Vollenweider et al. (1998) to calibrate in situ TRIX and an additional recent time series covering the period 2001–2012 to validate the model TRIX. The monitoring grid considers 21 sampling stations located along eight transects perpendicular to the coast: 19 stations are coastal, extending from 500 m to 10 km offshore, while two stations are at a 20 km distance, sampling an open shelf regime. All the stations are monitored weekly. ARPAE-Daphne divided the monitored area into three subareas (area A, B, and C in Fig. 1) on the basis of the hydrological and trophic conditions (Montanari et al., 2006). Area A is located immediately south of the Po delta and is directly affected by river runoff and nutrient load (see Po River in Fig. 1); it is therefore characterized by enhanced primary production. Area B is a transition area, while area C is characterized by hydrographical conditions mainly governed by the large-scale basin circulation.
The in situ TRIX was calculated for each station (using surface values of Chl, DO, DIN, and TP) and averaged over the eight transects and the three subareas. At the Porto Garibaldi and Cesenatico transects (see Fig. 1), TRIX was calculated with and without the open-shelf stations.
Lower (
The model data used in this study were produced by the three-dimensional coupled circulation–biogeochemical model consisting of the Princeton Ocean Model (POM; Blumberg and Mellor, 1987) and the Biogeochemical Flux Model (BFM; Vichi et al., 2007). The model was implemented in the Adriatic Sea at a horizontal resolution of about 2 km, and 27 sigma layers defined the vertical resolution (Clementi et al., 2010).
BFM is a complex lower trophic marine biogeochemical model. It is a biomass-based model, designed to simulate the main marine biogeochemical fluxes through a description of the ecological functions of the producers, decomposers, and consumers and their specific trophic interactions in terms of basic elements (carbon, nitrogen, phosphorous, silicon, and oxygen) flows. The biological constituents of the model are organized into chemical functional families (CFFs) and living functional groups (LFGs). CFFs are divided into organic (living and non-living) and inorganic compounds, which are measured in equivalents of major chemical elements or in molecular weight units. BFM receives information from the hydrodynamic model regarding temperature and salinity in order to calculate oxygen saturation.
The simulations were carried out for the period 1980–2010. Nutrients,
oxygen, and chlorophyll values were extracted at the model grid points nearest to
the in situ sampling stations (grey shaded areas in Fig. 1). The model TRIX
was then defined with
Pearson's correlation values between in situ TRIX and the TRIX
calculated from model simulations (
Comparison between in situ TRIX (red line) and model TRIX
(blue line) averaged over the three study areas (A, B, and C). Model TRIX was
calculated using
Figure 2 shows the comparison of the in situ and model TRIX considering the
upper (
Area A has the highest correlation values since it is the most eutrophic
area, exhibiting the highest TRIX values (
In all the three study areas, TRIX increases between 2008 and 2010. Area A shows
values above 6 (bad water quality conditions; see Table 1) in late
winter–early spring and late summer–early autumn (Fig. 2). Areas B and C are
characterized by TRIX values that indicate mediocre (5
Percentage contributions of the four variables (Chl, DO, DIN, and TP) calculated as in Eq. (4) to the in situ TRIX (colored areas) and the model TRIX index (continuous lines with different symbols) in the three study areas (A, B, and C).
Efficiency coefficient calculated from in situ data (red line) and model simulations (blue line) in the three study areas (A, B, and C).
The percentage contribution of each variable (Chl, DO, DIN, and
TP) to the TRIX index as a function of time was calculated in the three
subareas for in situ data and model simulations (Fig. 3). The percentage is
defined as
The model generally overestimates the dissolved oxygen. The mean DO percentage contribution calculated from model simulations corresponds to 40.86 %, while the DO percentage contribution calculated from in situ data corresponds to 28.86 %. However, the percentage contribution of the simulated total phosphorous is generally lower (mean value: 13.10 %) compared to the in situ data (mean value: 28.55 %). The percentage contributions of the simulated chlorophyll (mean value: 15.21 %) and dissolved inorganic nitrogen (mean value: 30.82 %) were in the same range as the in situ data (mean values: 17.37 and 25.21 % for chlorophyll and inorganic nitrogen, respectively). This means that the estimated model TRIX reaches similar values (Fig. 2) to the in situ ones for different reasons, which can be explained by the efficiency coefficient.
Pearson's correlation values between the in situ TRIX and the
model TRIX (
Comparison between in situ TRIX (red line) and model TRIX
(blue line) averaged over six transects (Lido Volano, Casalborsetti, Ravenna,
Lido Adriano, Rimini, and Cattolica) comprising the ARPAE-Daphne monitoring
sampling program. Red and blue shaded areas correspond to average values
The efficiency coefficient calculated from in situ data range between
Comparison between in situ TRIX (red line) and model TRIX
(blue line) averaged over two transects (Porto Garibaldi and Cesenatico),
including stations up to 10 km distance from the coast (coastal,
Fraction differences between the TRIX calculated from simulated data
using
Monthly averages of the model TRIX for the Adriatic and upper Ionian Sea.
The TRIX index calculated as averages of each monitoring transect (Lido
Volano, Casalborsetti, Ravenna, Lido Adriano, Rimini, Cattolica, Porto
Garibaldi, and Cesenatico, in Fig. 1) instead of averages over the areas A,
B, and C provides indications that are consistent with the previous results (see
Fig. 5). The Pearson correlation values relative to each transect are
reported in Table 4 for all the transects (the
A preliminary conclusion is that model TRIX agrees, within the 2-standard-deviation range, with the corresponding in situ index. In addition the model TRIX estimates appear more coherent with in situ estimations for trophic conditions characterized by a tendency towards eutrophy.
A sensitivity analysis was carried out by comparing the TRIX index calculated
only with the coastal stations, up to 10 km distance from the coast (Fig. 6a
and b), and the TRIX index computed also considering the two offshore
stations, situated at 20 km off the coast for the Porto Garibaldi and
Cesenatico transects (Fig. 6c and d). The Pearson correlation coefficient
decreased for the Porto Garibaldi transect and did not change for the
Cesenatico transect (Table 4), but all
Another sensitivity analysis was carried out by calculating the TRIX from
simulated data using
The correlation between the in situ TRIX and the TRIX calculated from model
simulations (model TRIX and model TRIX 1, 2, and 3) varied depending
on the time period considered (Table 3). Generally, area A was the
best-fitted area, with high correlation values for all the time periods
studied. Correlation values decreased in areas B and C when we considered
short-time-period data (5–10 years). However a high correlation was
observed, in all the three areas, between the in situ TRIX and the
model TRIX 1 calculated using
The results of the sensitivity analysis, focused on the assessment of the model capability to provide a useful TRIX index, suggest the use of simulated time series of at least 10 years to obtain the upper and lower limits for the state variables involved in the TRIX computation.
In all cases, model values are better correlated with in situ ones in eutrophic as compared to quasi-oligotrophic areas and in nearshore than offshore areas.
Finally, the model TRIX was computed for the whole Adriatic and Northern Ionian Sea. The monthly means computed for the time period 2001–2010 are shown in Fig. 8.
There is a sharp contrast between the coastal northwestern shores, the
southeastern shelves, and the open-sea areas. The northwestern Adriatic Sea
shows the highest TRIX values (
In this paper we have adapted the TRIX generic equation to an ecosystem
model simulation, and we have computed the TRIX over an entire sea basin
region. In order to calibrate the model index, all the index scaling
parameters were calculated from the model simulation itself. Each state
variable is scaled by the highest (
The analysis was based on a comparison of in-situ-data-derived TRIX and the model TRIX on the Emilia-Romagna coastal strip. First of all, the results indicated the generally significant potential skill of the model in replicating the observed index, provided that the TRIX scaling parameters are computed from the simulated data set itself. The comparison also indicated that in a statistical sense the model's replication skill decreases consistently with the trophic characteristics' tendency towards oligotrophic conditions. This is indicated by the decreasing correlation values from area A to areas B and C. This obviously merits further investigation as it is probably related to the main driver controlling the local environmental conditions (external input vs. circulation and vertical structure dynamics).
Despite the good correlation between model and in situ TRIX values, it is
clear that the model obtains such values with different proportions of the
four state variables, Chl, DO, DIN, and TP. In particular we observed that the model
generally underestimates the total phosphorous and overestimates the
dissolved oxygen. This model–data discrepancy leads to a general
underestimation of the model efficiency coefficient with respect to the in
situ data. However, both the model results and the in situ data showed that
the efficiency coefficient is low,
The sensitivity analysis to the extension and specific time series used to
evaluate the scaling parameters indicated that for time series longer than
10 years the results were insensitive to the (
When implemented in the whole Adriatic Sea basin scale, the model TRIX produced the sharp transition from the eutrophic oriented conditions of the coastal domain, to the oligotrophic conditions that characterize the pelagic domain.
Numerical simulations can therefore represent an important support for monitoring activities; i.e., they will allow the use of TRIX to be extended to much larger areas where in situ sampling activities are difficult to implement. However it is important to realize that in the future it will be desirable to use several numerical models instead of just one realization in order to associate model uncertainties with the model TRIX estimates.
The data sets on the simulated trophic index (TRIX) and the four state variables
comprising the index (chlorophyll
This research received partial support from the PON (Programma Operativo Nazionale) project “TESSA” (Tecnologie per la Cognizione dell'Ambiente a Mare), funded by the Italian Ministry of Education and of Economic Development, and by the EU FP7 project “PERSEUS” (Policy-Oriented marine Environmental Research for the Southern European Seas). Edited by: R. Archetti Reviewed by: two anonymous referees