Phytoplankton Dynamics of Mokolo and Mopa Ponds in Bertoua City (East-Cameroon)

This study aims to improve the understanding of algal community’s dynamics in response to different environmental factors in two dam ponds (Mokolo and Mopa) in the city of Bertoua (East-Cameroon). Physicochemical and biological analyzes were carried out monthly by direct sampling at the surface and using Van Dorn bottle at 1 m depth. The organisms were collected using transparent glass vials of about 500 ml and fixed with 2.5 ml of a lugol solu-tion, then analyzed using the Utermôhl method. Physicochemical analyzes show low transparency (<75 cm) of the ponds despite their shallow depth (≤150 cm), high levels of dissolved oxygen (>60%), BOD 5 (>30 mg/L) and chlorophyll “a” (>30 µg/L). These data made it possible to categorize the Mokolo and Mopa ponds as hypereutrophic with nitrogen as the limiting factor for eutrophication. Biological data show quite diversified ponds with 138 species identified in Mokolo Pond and strongly dominated by Diatoms with 2951 ind. representing 46% of the total abundance. In Mopa Pond, 147 species were identified, mainly represented by Chlorophyceae with 3629 ind. representing 52% of the total abundance. Azpeitia africana (Mokolo) and Eresmophaera gigas (Mopa) were the most represented taxa during the study. This study will have deduced that the structure and dynamics of algal communities are under the control of different factors or processes that interact simultaneously, namely ascending factors or bottom-up corresponding to nutrient resources and sunlight and descending factors or top-down that are exerted by grazing and active physiological substances produced by other algae that are known to influence phytoplankton.


Introduction
Phytoplankton is made up of all micro-organisms plant living in water column, unable to resist the current and capable of developing their own organic substance by photosynthesis, from solar energy, water, carbon dioxide and nutrient salts [1]. In aquatic ecosystems, the different species of algae are distributed according to their biological and ecological requirements. Also, the study of the environmental variables of a biotope and the species that colonize enables to determine the relationships between environmental factors and organisms, and to identify the ecological factors that are appropriate for each species [2]. Phytoplankton organisms are widely considered to be the first biological community to respond to anthropogenic pressures and are the most direct indicator of nutrient concentrations in the water column of all biological elements [3]. Their metabolism is dominated by the autotrophic lifestyle based on photosynthesis [4]. A possible imbalance of intrinsic and/or extrinsic origin due to the control of nutrient resources can affect water quality and lead to a modification of the structure of biological community dependent on these hydrosystems, favouring the proliferation of certain algal species known as efflorescence [5]. These blooms can have many economic, ecological and even health consequences, because the massive growth of some phytoplankton populations can pose a risk to fish and therefore to consuming populations [6]. This work aims to contribute to the understanding of the mechanisms governing the distribution of algal communities through the study of the phytoplankton structure of two ponds (Mokolo and Mopa) in the city of Bertoua (East Region) in relation with some abiotic parameters.

Presentation of the Studied Ponds
The city of Bertoua is located in the Department of Lom-and-Djérem, Eastern Region of Cameroon. The temperature is high all the year round and varies between 18˚C and 30˚C. Rainfall is relatively abundant (1500 to 2000 mm of rainfall per year) and the climate is subtropical with two seasons [7]. This study focuses on two dam ponds: Mokolo and Mopa in Bertoua city.

Mopa Pond
Mopa Pond is a dam pond located in the Nkolbikon district on the outskirts of the city of Bertoua, with geographical coordinates 04˚34'408'' North latitude and 013˚39'188'' East longitude, with an altitude of 650 m ( Figure 1). It is an abandoned pond, not maintained, characterized by many plants mainly the species Nymphaea lotus and Pistia stratiotes which cover the water surface hinding a better penetration of light. The main sources of pollution in the pond are the strong macrophytic vegetation, trees and shrubs that surround the pond, as well as the plantations and dwellings located in the watershed.

Data Collection
Sampling was carried out from March 2016 to April 2017 followed a monthly frequency with surface and 1 m depth sampling for physicochemistry and biology. The movements on the ponds were possible using an inflatable Zodiac MR II.

Physicochemical Analysis
Samples for surface physicochemical analyzes were collected directly at the surface using polyethylene vials, while at 1 m depth, these samples were collected using a 6 L Van Dorn bottle. The physicochemical parameters measured in the field during this study were temperature measured using a 1/100 th degree mercury column thermometer, transparency (Zs) measured using a 30 cm diameter black and white Secchi disc, depth measured using a weighted and graduated rope and dissolved oxygen measured using a HACH HQ14d oxymeter. The parameters measured in the laboratory included the nutrient salts (NO − 3 , NO − 2 , NH + 4 , PO 3− 4 ) measured using the colorimetric method with the HACH/DR 2010 spectrophotometer, the BOD 5 measured by respirometry using a LIEBHERR brand BOD meter and the chlorophyll "a" content measured by the Lorenzen spectrophotometric method [8]. These physicochemical analyzes were carried out using the AFNOR method [9].
Nitrogen or phosphorus or both nutrients elements will be limiting respectively if the N/P ratio is less than, greater than or equal to 16. To characterize the trophic state of the ponds, the system developed by the O.C.D.E. [10] and widely used internationally (Table 1), has been used.

Biological Analysis
Phytoplankton organism was collected by direct sampling at the surface and using a Van Dorn bottle at depth and then transferred to clean, transparent 500 ml glass vials and fixed with 2.5 ml of a lugol solution. After 48 hours of sedimentation, the supernatant was gently removed and the sub-sample of approximately 5 ml denser was preserved. After homogenization, 1 ml of the sub-sample was pipetted and observed in a Sedgewig-Rafter counting cell with an inverted microscope (Olympus CK2). The count was duplicated to minimize the risk of error and the identification of at least 400 individuals per sample was recommended for an accuracy of +/− 95% [11]. Due to the richness of some samples in particles and organisms, a dilution to 1/10 th or 1/20 th with distilled water was essential to facilitate enumeration. The count was carried out using an OLYMPUS CK2 inverted microscope with a 10X objective, with scans from left to right of the surface of the counting cell with alternating transects. Taxa have been identified  [14], as well as books and publications on phytoplankton taxonomy from Couté and Iltis [15]; Kemka [16] and Couté and Perrette [17]. The identified organisms will be grouped according to morphological, cytological, biochemical and reproductive criteria into 8 main phytoplankton Classes namely: Chlorophyceae, Chrysophyceae, Cryptophyceae, Cyanophyceae, Diatoms, Dinophyceae, Euglenophyceae and Xanthophyceae.
The determination of the density was calculated by the following formula: with D = total headcount/litre; S = area of the counting cell (100 mm 2 ); N i = counted number of individuals of a species; s = area of the total counted field (200 mm 2 ) and v = volume of sedimented sample (5 ml). The Hill diversity index [18] has been used to highlight the overall stands diversity and their degree of organization [19] with a maximum diversity of 1 and a minimum diversity of 0 based on the formula: with 1/D = inverse of the Simpson index and e H' = exponential of the Shannon and Weaver index. Piélou's regularity index (E) which varies between 0 and 1, makes it possible to study the regularity of the distribution of species and reflects the quality of organization of a stand [20] according to the equation: with H' = Shannon and Weaver diversity index and S = specific richness. The frequency of occurrence provides information on the environment preferences of a species and has been used to count the number of times it appears in samples [21] using the formula: with F i = number of records containing the species and F t = total number of samples taken. Depending on the value of the frequency, five categories of taxa are defined according to the classification of Dufrêne and Legendre [21]: F = 100%: Omnipresents taxa; 75% ≤ F < 100%: Regulars taxa; 50% ≤ F < 75%: Constants taxa; 25% ≤ F < 50%: Accessories taxa and F < 25%: Rares taxa. The Rank-Frequency Diagram (RFD) was used to assess the maturity of the stand and the succession of the development stages. The Canonical Correspondence Analysis (CCA), used to determine the abiotic factors influencing abundance of taxa.

Physicochemical Parameters
The ponds studied have relatively high temperatures, low water transparency and shallow depth, medium oxygenation, high levels of nutrients, organic matter and chlorophyll "a" ( Table 2). All these characteristics show a very poor water  quality of the ponds. The average of N/P ratio are very low comparatively to Redfield standard ratio (N/P = 16). The physicochemical parameters measured in this study didn't vary significantly (P > 0.05) from the surface to the depth showing a homogeneous quality of water. Only the water temperature varies significantly (P < 0.05) between the two ponds during the study period ( Table 2).  Families, 28 Orders and 5 Classes (Table 3).

Structure of Phytoplankton Groups
In Mokolo Pond, the specific richness was dominated by Chlorophyceae group  with an average at the surface of 6 ± 4 species and at depth of

Data Analysis
The Rank-Frequency Diagram curves plotted across the ponds have all the same Open Journal of Ecology convex shape representing stage 2 of the evolution of planktonic communities (Figure 4(A)). The structure of the frequency of occurrence is the same in the two ponds studied. Rares taxa were the most dominant (>75%), followed by accessories taxa (>15%) and constants taxa (>4%). Regulars taxa (<2%) and omnipresents taxa (<1%) were poorly represented (Figure 4(B)). The Hill diversity index in Mokolo Pond at the surface changes from 0.89 bits/ind. to 0.98 bits/ind. with an average of 0.95 ± 0.02 bits/ind. and at depth from 0.85 bits/ind. to 0.97 bits/ind. with an average of 0.92 ± 0.04 bits/ind. This index varies in Mopa Pond at the surface from 0.87 bits/ind. to 0.98 bits/ind. with an average of 0.95 ± 0.04 bits/ind. and at depth from 0.9 bits/ind. to 0.98 bits/ind. with an average of 0.94 ± 0.03 bits/ind. (Figure 5(A)). The regularity of Pielou was low in the ponds going into Mokolo Pond at the surface from 0.35 to 0.58 with an average of 0.47 ± 0.06 and at depth from 0.3 to 0.52 with an average of 0.41 ± 0.07. In Mopa Pond, this index varies at the surface from 0.32 to 0.58 with an average of 0.47 ± 0.08 and at depth from 0.35 to 0.57 with an average of 0.45 ± 0.08 ( Figure 5(B)). The Hill diversity index and the Piélou regularity doesn't vary significantly (P > 0.05) from the surface to the depth and from one pond to another in the study period.   croorganisms that release humic acid, which contributes to reduce the pH values [25]. The high levels of chlorophyll "a" in ponds may be due to the high levels of nutrients (nitrogen and phosphorus) that can boost algal productivity. In this regard, Wurtz [26] argues that nutrients stimulate the growth of phytoplankton organisms, which are then used as food for microscopic animals such as zooplankton.

Physicochemical Parameters
The

Phytoplankton Dynamics
The relatively high specific richness of phytoplankton recorded in Mokolo (138) and Mopa (147) ponds is due to the anthropisation of the watershed of the ponds with organic and mineral materials containing high levels of nutrients and dissolved substances that promote the rapid and continuous growth of algae and aquatic plants. In this regard, Findlay and kling [28] point out that the low water volume associated with high nutrient levels is favourable to the development of phytoplankton organisms. The high abundance of Diatoms in the two ponds despite their low motility could be explained by the cosmopolitan nature of these organisms with a strong capacity to adapt to variations of environmental conditions and to various environments. Their distributions throughout the water column are related to winds and rains that generate strong turbulence, allowing the resuspension of Diatoms and other algae that tend to sink because of their densities, which are always slightly higher than the water, and to be found in the euphotic zone when they are below [29]. The high abundance of Chlorophyceae is explained by the very high levels of nitrogen elements recorded during the study period, making this nutrient the limiting factor in all the ponds studied. In this regard, Berube et al. [30] [31]. The average abundance of Cyanophyceae is thought to be due to the fact that these organisms prefer environments where phosphorus is limiting and are generally found in shallow hydrosystems rich in nutrient [32]. The low abundance of Dinophyceae and Chrysophyceae is believed to be due to the particular flowering conditions of the organisms belonging to these two groups. Dinophyceae blooms are generally associated with salty or poor nutrient environments [33] while Chrysophyceae tend to form blooms in oligotrophic environments [34]. Algae densities have been high in the ponds, making them highly productive. These high densities result from the combined action of the different elements of abiotic origin on the development of microalgae. For example, Carpenter et al. [35] reports that dissolved organic matter can have both positive and negative effects on phytoplankton growth. The significance of these effects may vary depending on the source of this dissolved organic matter and the composition of the phytoplankton community. Phytoplankton density and biomass are in fact controlled mainly by nutrient availability [36].
The analysis of dominants taxa shows that Volvox tertius, Eresmophaera gigas, Pleurotaenium trabecula, Microcystis aeruginosa, Aphanocapsa incerta and Azpeitia africana are bioindicator species most characteristic of the hypereutrophic state of these ponds during the study period. The frequency of occurrence follows the same structure in all the ponds studied with a very high abundance of rare taxa about 80% in all ponds and a very low presence of regular taxa about 2% and omnipresent taxa (<1%). These data can be explained by the constant and irregular nutrient inputs to ponds that do not provide standard conditions for the growth and sustainability of a greater number of species. The convex shape of the Rank-Frequency Diagram curves representing stage 2 of evolution demonstrates the maturity of the phytoplankton communities in the ponds, which are fairly well diversified with a small number of species whose dominance is much higher than the other species [37]. These strong and constant recolonization of the environment by "r" strategy species that rely on reproduction with a high growth rate and very high mortality, adapting to unstable, unpre-

Conclusions
The ponds studied (Mokolo and Mopa) are rich in nutrients, with high content of chlorophyll "a" and low transparency. These characteristics classify ponds as hypereutrophic. These ponds are highly diversified and are not in balance, because they are dominated by a small group of species. This study has shown that the dynamics of algal communities depends on ascending factors or "bottom-up" corresponding essentially to nutrient resources, CO 2 contents and sunshine, which are the main photosynthetic factors that determine species succession, and descending factors or "top-down" that are essentially exerted by grazing and physiologically active substances produced by other algae that are known to have an influence on phytoplankton.
The rehabilitation of these hydrosystems involves reducing exogenous nitrogen inputs to the ponds in order to control eutrophication, mowing and gathering aquatic plants that proliferate in the ponds in order to maintain good oxygenation of water, as well as cleaning the sludge in order to limit the release of nutrients from the sediments to the water.