# Gill Histopathology as a Biomarker for Discriminating Seasonal Variations in Water Quality
## 3 Results and Discussion  
### Water Quality  
Water quality analysis displayed certain differences in several parameters between seasons (Table 1). Seasonal differences were mainly encompassed by levels of electroconductivity, ammonia, chlorides, Cu, Fe and Pb. According to WHO guidelines [21], only the levels of Fe from the Tamis River were above the guideline limit in both seasons, while Pb values were just below the limit during season one. On the other hand, according to maximum admissible values proposed by [PERSON] et al. [22], levels of Fe, Cu and chlorine from the Tamis River were elevated in both seasons and could potentially cause tissue damage in cyprimid fish. Chemical analysis of water can provide valuable data for interpretation of observed histopathological alterations by pointing out possible pollutants which could induce observed alterations [10]. [PERSON] Histopathological Observations  
Microscopic examination of bream gills demonstrated alterations in the gill structure (Figure 2)[PERSON] Epithelial hyperplasia (Figure 2A), hypertrophy (Figure 2B), epithelial lifting (Figure 2C) and necrosis (Figure 2D) were the most pronounced alterations detected in the present study. In some instances, hemorrhage (Figure 2A) or lymphocyte infiltration indicative of mild inflammation were also observed (Figure 2B). These alterations are commonly observed in histopathological studies. [PERSON] [8] has hypothesized that epithelial hypertrophy and necrosis are more frequently the result of heavy metal exposure, which could be correlated with the higher Fe and Cu concentrations observed in the water of the Tamis River. However, ascribing the cause of certain gill histopathological alterations to specific environmental conditions or pollutants is not possible, as gills mostly react in a generalized way; therefore, many different factors can cause the same structural alterations [8; 23; 24]. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Parameter** & **Season 1** & **Season 2** & **WHO [21]** \\ \hline Water temperature (\({}^{\circ}\)C) & 8.2 & 14.8 & - \\ Water transparency (mm) & 150 & - & - \\ Suspended matters (mg/L) & 39 & 35 & - \\ Dissolved oxygen (mL/L) & 10.4 & 9.5 & - \\ Alkalinity (mmol/L) & 1.7 & 1.6 & - \\ pH & 7.6 & 7.5 & - \\ Electroconductivity (\(\mu\)S/cm) & 235 & 579 & - \\ Ammonia (mg/L) & 0.2 & 0.05 & No guidelines \\ Nitrates (mg/L) & 0.77 & 0.56 & - \\ Nitrites (mg/L) & 0.01 & 0.013 & - \\ Orthophosphates (mg/L) & 0.028 & 0.032 & - \\ Chlorides (mg/L) & 12 & 7.3 & No guidelines \\ Sulphates (mg/L) & 23 & 25 & - \\ Fe (mg/L) & 1.24 & 1.78 & 1 \\ Mn (mg/L) & 0.07 & 0.05 & 0.5 \\ Zn (mg/L) & 0.04 & 0.04 & 5 \\ Cu (mg/L) & 0.02 & 0.007 & 2 \\ Total Cr (mg/L) & 0.002 & 0.002 & 0.05 \\ Pb (mg/L) & 0.009 & \textless{}0.001 & 0.01 \\ Cd (\(\mu\)g/L) & 0.2 & 0.2 & 3 \\ Hg (\(\mu\)g/L) & 0.1 & \textless{}0.1 & 1 \\ Ni (mg/L) & 0.006 & 0.004 & 0.02 \\ As (mg/L) & 0.001 & \textless{}0.001 & 0.01 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Water quality parameters for the Jaša Tomic site in different seasons [18]. Furthermore, even though parasitic examination was not in the scope of the study, a very small number of _Triclodina_ sp. were found in a few fish from both seasons. Structural alterations mainly arise from two principal reasons: either as a defense mechanism, or as a result of direct damage cause by environmental factors/xenobiotics. Epithelial hyperplasia, hypertrophy and lifting are considered to be defense mechanisms of gills as the former decreases the respiratory surface, while the latter two increase the diffusion distance, thus hindering xenobiotic uptake [7, 8]. These alterations are reversible, meaning that the gill tissue is expected to recover once exposure to irritants ends [20]. On the other hand, alterations such as necrosis, aneurism or hemorrhage are considered to be a result of xenobiotic action, and are generally considered to be irreversible.