Macrophytes as a tool for assessing the trophic status of a river: a case study of the upper Oum Er Rbia Basin (Morocco)

Aquatic macrophytes are a characteristic constituent of rivers and streams due to their multidimensional role in such lotic systems (Sukhodolova et al. 2017; Zhang et al. 2019). They are defined by their ability to increase the diversity of ecological niches and they provide a habitat structure for other taxonomic groups such as periphyton, macroinvertebrates, fish and birds (Altena et al. 2016; Celewicz-Goldyn & Kuczynska-Kippen 2017; Rodrigues et al. 2019; Roussel et al. 1998). Through the process of nutrient exchange between the sediment and the water column, macrophytes can affect flow velocity (O’Hare 2015; Old et al. 2014), water quality and the nature of substrate (Cotton et al. 2006; Madsen et al. 2001). The ability of macrophytes to grow, survive and reproduce under prevailing environmental stressors has increasingly made them the subject of research in various countries with the objective to develop multiple metrics to assess the ecological status of their hydrosystems. Several countries have developed their own macrophyte indices to assess the trophic state corresponding to trophic classes determined by the content of ammonium and orthophosphate in lotic waters (Carbiener et al. 1995): Mean Trophic Rank (MTR) in the United Kingdom (Dawson et al. 1999; Holmes et al. 1999), Trophic Index with Macrophytes (TIM) in Germany (Schneider & Melzer 2003) and Macrophyte Biological Index for Rivers (IBMR) in France (Haury et al. 2006). These indices are based on the hypothesis that the distribution of macrophytes in lotic waters depends mainly on phosphate and/or nitrogenous compounds. The Macrophyte Biological Index for Rivers (IBMR) is based on the study of macrophytes to determine the trophic status and organic pollution in French rivers, a method that has been used by several countries in the Mediterranean region such as Italy, Portugal, Spain, Greece and Slovenia (Szoszkiewicz et al. 2006). Despite Morocco's location in the Mediterranean region, only a few studies have been focused on assessing the ecological status of watercourses using macrophyte metrics (Bentaibi et al. 2017). The present study attempts to apply the IBMR method to assess the trophic status of the upper Oum Er Rbia basin (center of northern Morocco) in order to test its suitability in the Moroccan context and to investigate the structure of macrophyte communities and their relationships with river physicochemical parameters.

Physicochemical data from the water analysis show that all rivers were oxygenated throughout the study period, which mainly reflects a high metabolism of aquatic vegetation in an open hydrosystem (Hauer & Lamberti 2017). The Amengous River and the Srou River were characterized by high nutrient levels. The downstream sites of the Oum Er Rbia River were exposed to an increase in nutrient levels. Nonetheless, these values remain high, permitting the rivers to be classified into trophic levels, varying from mesotrophic to eutrophic (Environmental Protection Agency 2000; Haury & Peltre 1993; Nisbet & Verneaux 1970; O’Hare et al. 2018). These results are confirmed by the presence of species characteristic of high trophic levels such as Myriophyllum spicatum, Zannichellia palustris and Groenlandia densa (Fabris et al. 2009; Robachl et al. 1996; White & Hammond 2008), which together form a plant community in the Amengous River. Furthermore, the presence and sometimes abundance of ubiquitous Chlorophyceae, such as Cladophora sp. and especially Vaucheria sp., indicate a high trophic level and high content of ammonium and phosphate (Thiébaut & Muller, 1999). The high values of electrical conductivity (EC) in the Oum Er Rbia River and the Srou River are mainly due to various saline springs that feed these watercourses. The pH is alkaline, which is typical for eutrophic waters (López-Archilla et al. 2004). The temperature is a determining factor for the development of these algae species (Vaucheria sp., Cladophora sp. and Spirogyra sp.), which tend to grow in a range of 15–28°C (Pikosz & Messyasz 2016; Sheath & Cole 1992). In addition to temperature, various saline springs feeding the Oum Er Rbia River increase water mineralization and favor the growth of Ulva sp. The marly-limestone nature of the Oum Er Rbia riverbed and the increase in the trophic level at the downstream sites favor the development of some species (e.g. Zannichellia palustris and Stuckenia pectinata) that are characteristic of eutrophic marly-limestone environments (Haury et al. 1998). At Sru01 (downstream), the substrate is enriched with nutritive matter and the water flow is reduced, favoring the occurrence of two helophyte species (Typha latifolia and Typha angustifolia), which accumulate nutrients in their tissues and ensure the elimination of organic pollutants (Westlake et al. 2009). This explains the positive correlation between these helophytes and NO₂-N and COD_Mn. The negative correlation between some species and NH₃-N may be due to the alkaline reaction of water, which makes NH₃-N a plant growth inhibitor (Kohler 1976; Litav & Lehrer 1978; Wang et al. 2008; Zhu et al. 2018). The amount of dissolved oxygen in the water results from the balance between the dissolved oxygen supply associated with the photosynthesis activity and the surface exchange with oxygen in the air as well as the oxygen consumption necessary for respiration, biochemical and chemical reactions to degrade organic matter. The high concentration of dissolved oxygen (DO) recorded in the Amengous River can be explained by metabolic activities (photosynthesis) of the higher biomass of aquatic plants in this river (Barbe 1984; Sukhodolova et al. 2017). Our survey showed that some species were more likely to be present at high concentrations of nutrients (e.g. Persicaria amphibia), whereas some species characteristic of eutrophic environments (e.g. Myriophyllum spicatum, Groenlandia densa, Zannichellia palustris) were more closely correlated with nutrients in water. This can be explained by the effect of other factors such as water temperature, flow velocity and mineralization of water, which prevent nutrient assimilation from water and promote nutrient uptake from the sediment by macrophyte root systems (Baldy et al. 2007; Onaindia et al. 2009).

The obtained IBMR values indicate that the Oum Er Rbia River and its tributaries have a very high, high and moderate trophic state, which confirms the results discussed above concerning the physicochemical parameters and the distribution of macrophytes. Our correlation analysis shows that the IBMR index depends not only on the nutrient parameters but also on other abiotic parameters such as water temperature (WT), electrical conductivity (EC), which explain the distribution of macrophytes more effectively than nitrate (NO₃-N) and nitrite (NO₂-N) in the upper Oum Er Rbia basin. Our results support other studies carried out in several countries (Demars et al. 2012; Kargioglu et al. 2012; Manera et al. 2014; Özbay et al. 2019). The results also showed a positive correlation between the IBMR values and nitrate (NO₃-N). This may be due to the nitrification activity controlled by environmental factors such as dissolved oxygen. Sometimes, under environmental pressure, macrophyte species prefer different forms of nitrogen in water (Hauer & Lamberti 2017), which may partly explain the apparent correlation between the IBMR value and nitrogen compounds. We note that phosphate (PO₄-P) and ammonia (NH₃-N) have no significant relationship with the IBMR value, which raises a serious question about the reliability of the IBMR index for the detection of pollution related to phosphate (PO₄-P) and ammonia (NH₃-N) concentrations in the studied rivers. It appears, however, that non-nutrient parameters in the upper Oum Er Rbia basin may play an important role in determining the distribution of macrophytes even in rivers that are exposed to eutrophication and have relatively high concentrations of nutrients, which may cause the IBMR to be a misleading index in some cases.

This study is the first in the upper Oum Er Rbia basin employing a macrophyte metric such as the Macrophyte Biological Index for Rivers (IBMR) to assess the trophic state of rivers. Our results provide new data on the physicochemical parameters controlling the distribution of macrophytes in the Moroccan river system, as well as on the role played by macrophytes as a sensitive indicator of eutrophication of watercourses. Even though the IBMR index showed encouraging results regarding the trophic state of the studied rivers, the correlation between this macrophyte metric and each hydrochemical parameter showed that the IBMR is more related to abiotic parameters than nutrients. These results allow us to pose a serious question concerning the suitability and reliability of the IBMR for the assessment of pollution related to phosphate and ammonium concentrations in our hydrosystems. Finally, it is necessary to develop an indication method for Moroccan rivers, which would take into consideration all species characteristic of our watercourses and which would include different environmental factors that characterize our rivers.