The coastal of Kribi is located on the edge of the Gulf of Guinea, in the South Cameroon Region and in the Ocean Department [22].

In the region experiences an equitorial climate of Guinean type characterized by the existence of four seasons, 02 dry seasons (from December to March for the long season and from July to August for the short season) and 02 rainy seasons (from September to November for the long season and from April to June for the short season) [23]. The average daily duration of sunstroke varies from 8 to 10 hours all year round [23]. Average daily maximum temperatures range from 25˚C to 33˚C while average daily minimum temperatures range from 15˚C to 22˚C [23]. As for rainfall, it reaches monthly values of 504 mm in September.

The vegetation is made of marsh forests, mangrove forests and evergreen forests creating a specific environment at the beach levels, making them attractive to tourists [24]. The hydrology is dominated by the Atlantic Ocean whose tidal range is low with small waves. The presence of the Lobé and Kiénké rivers is remarkable, as are the Ngoyè, Abondé, Wamié, Nzami, Bidou rivers, which receive household waste and wastes from agroindustrial companies [24].

The sampling points were chosen based on their economic, tourist and industrial importance (Table 1). Thus, 08 sampling points (Figure 1) were selected, including 04 in urban areas and 04 in peri-urban areas. In the urban environment, the points chosen consist of two beaches, a hotel site and an artisanal net fishing site (Figures 2(A)-(C)). In the peri-urban area, on the other hand, the points selected consist of a fish market, an artisanal fishing site, a beach (Figure 2(D)) and the northern limit of the deep-water port.

After determination of the euphotic layer using a Secchi disc, 03 sampling levels were retained, the first on the surface (surface-0.5 m) and the other two deep in the trophogenous and tropholytic layers. Sampling in the deep layers was made possible by a 6 L bottle of van dorn.

Water sampling for measurements of abiotic variables was carried out at the 03 predefined levels. The water taken made it possible to make a composite in a seal of 20 L previously washed and rinsed with water from the sample. In the composite, abiotic factors such as temperature, pH, conductivity, salinity, TDS, were directly measured using a Waterproof TESTER multiparameter and dissolved oxygen using a HACH HQ30d oxymeter. For water samples for laboratory analysis, 1000 cc of the composite was taken from a 1000 cc polyethylene vial and stored in a refrigerated chamber until the laboratory. In the laboratory, parameters such as color, Suspended Solids, nitrites, nitrates, orthophosphates were measured using the HACH DR 3900 spectrophotometer following standard analytical protocols and recommendations of Rodier et al. [25]. Alkalinity was measured by titration.

A total of 90 L of water, 30 L per level, were also carried out at the 03 predefined levels and then filtered using a plankton sieve with a 20 μm mesh opening. The retentât obtained, was directly fixed adding lugol. In the laboratory, the sample was left to settle for 48 hours and then the supernatant was siphoned off and the resulting cap was sedimented in a 25 mL sedimentation tank. After homogenization of the sediment, 1 mL was pipetted and then observed under the Olympus CK2 microscope at 200× magnification using the Sedgewick-Rafter counting cell with left-right scans alternating transects of the counting cell. Taxa have been identified using specialized literature by Okoloddkov [11]; Evagelopoulos [26]; Phan-Tan et al. [27]; Al-Kandari et al. [28]; Komoé [29]; Lakkis [30]. The different morphotypes were photographed using an OMAX ToupView eye camera and those of the taxa of the genera Ceratium in particular were processed using Photoshop 2020 image processing software. The densities of the different taxa identified were calculated from the following formula:

D = N i × S × 1000 / ( v × s )

with D = density (ind./L); S = area of the counting cell (1000 mm²); N_i = number of individuals counted; 1000 = conversion factor in litres; s = field area of the cell counted and v = volume of sediment (25 mL).

The frequency of occurrence (F) expressed as a percentage, which provides information on the constancy of a species or taxon in a given habitat, was calculated using the formula of Dufrêne & Legendre [31]. A distinction is made between ubiquitous (100%), regular (75% to < 100%), constant (50% to <75%), accessory (25% to <50%) and rare (<25%) [32]. This index is based on the presence/absence matrix and is calculated by the relationship:

F = P i × 100 P t

with: P_t = the total number of samples taken

The influence of abiotic variables on the organisms studied was tested using SPSS 20.0 software. Nonparametric tests made it possible to establish the distribution of the data and the Spearman correlations made it possible to show the links between the abiotic factors and the different taxa identified. To test the effect of the space represented by the stations and the seasons, the ANOVA test was used for each of the parameters.

In order to give a synthetic image of all the affinities between the different populations of the genera Ceratium and Protoperidinium on the one hand, then to highlight the so-called correspondence relationships between taxa and sampling points on the other hand, Factor Analyses of Correspondences (AFC) were carried out whit the R software version 4.1. The least frequent and least dominant taxa were excluded since their share of information is very small since the AFC requires the simplification of the table of basic data especially when it comes to a huge number of taxa.

All data from the abiotic variables, diversity and density of the genera studied were grouped into seasons according to the climatic breakdowns of [23]. The Box Plots to illustrate the seasonal variation of abiotic factors were made using GraphPad Prism 8.0.2.

Variations in abiotic variables (temperature, pH, salinity, nitrates, phosphates, Suspended Solid) were moderate along the stretch of coast studied throughout the study period and significant with differences in concentration between different seasons.

The level of temperature variation is related to the extent of the section studied and the measuring range. The average temperature value (29.51˚C) recorded during this study shows an increase in temperature in the Kribi coast over the years compared to those obtained by similar studies [33] in 2004, 2005 and 2006 (27.2˚C; 27.4˚C; 28.6˚C) respectively. This difference observed between the two studies could be justified by the frequency of measurements or the effects of climate change on temperature. A difference that would be justified by the fact that the measurements were made on an ad hoc basis from one station to another and at irregular times at times when the sun was at its zenith [34].

The levels of water pH variation recorded during this study reflect a pH illustrating a halin domain. Sudden changes in pH have often caused many disturbances in the biological compartment. Although flowers are generally considered non-toxic to marine invertebrates, they have been suspected of being related environmental damage through pH changes, mild reduction, and oxygen depletion, which contributed to the deaths of various phytoplankton, fish, zoobenthos, shellfish [35].

The slight decrease in salinity observed at P4 compared to the other points could be the result of an influence of the fresh waters of the Lobé. Overall, according to a report by MINEPN [35], the increase in salinity in some places may be due to a low supply of fresh water (for example in the Kribi region compared with that of Limbé close to Wouri Estuary). However, it has been reported that the low salinity of the water such as that recorded during the long rainy season favors the proliferation of dinoflagellates especially the benthic species [36].

The Suspended Solids were generally low in the different points ranging from 4.69 to 8.46 mg/l with an average of 06.36 ± 1.43 mg/L. The averages observed were generally lower than those of the Suspended Solids obtained upstream of a coastal river, the Kienké in Kribi [37]. The low Suspended Solids are linked to the phenomenon of clogging and sedimentation in the marine environment.

Nitrites, nitrates and orthophosphates recorded during this work, they were higher than those recorded in 2006 by Krakstad et al. [33] in a study that covered 07 months with a monthly measurement frequency (<0.003; 0; 0.5 mg/L for nitrites, nitrates and orthophosphates respectively). The increase of nitrates (1.52 ± 0.44) and orthophosphates (1.84 ± 0.2) concentration recorded during this study could be come from mainly industrial, domestic, hotel, landing stages and fish markets. The high concentrations observed at P2, P5 and P8 would testify to the influence of the site activities on the enrichment of the environment with these nutrients. According to Carpenter [7] and Diaz et al. [8], the bioavailability of nutrients, mainly phosphorus and the different forms of nitrogen in excess concentrations could be the root cause of the algal bloom attributed to the eutrophication of water bodies.

The average temperature values of water, nitrates and orthophosphates compared to previous work showed an increase in temperature on the coast over the years and nutrient enrichment. Temperature and dissolved oxygen are the abiotic factors that have more positive and significant influence; negative and significant respectively on most of the species recorded.

A total of 33 taxa including 15 belonging to the genera Ceratium and 18 to the genera Protoperidinium. This diversity is low compared to the results of the work of Lakkis [29] in Lebanese marine waters and the Levantine basin where 54 species of Ceratium and 32 species of Protoperidinium have been identified; Okolodkov [12] in the Sistema Arrecifal Veracruzano Park, in the southern Gulf of Mexico, which has recorded 33 species belonging to the genera Ceratium. Koéme [28] identified 21 species of Ceratium and 04 species of Protoperidinium in the Grand-Lahou lagoon complex in Côte d’Ivoire. This difference of diversity observed could be related to the duration of each study, the specific characteristics and the climatic conditions prevailing in the study areas. The spatio-temporal variation in the context of this work could also be justified by the specific characteristics of each sampling point, the climatic conditions specific to each season and the geographical distribution of species.

Amongst all the species recorded, Ceratium furca was the only regular species. A similar finding was made by Lakkis [29] in Lebanese waters. This species as well as Ceratium fusus and Ceratium tripos although generally incidental and rare in the context of this work have been recognized by several works as a harmful species. Indeed, according to Schmidt and Schaechter [38], the species belonging to this genus Ceratium, specifically Ceratium tripos, Ceratium furca, and Ceratium fusus were able to produce the largest flowers of dinoflagellates due to their large size which takes into account that a limited number of cells have a significant impact. The red-brown flowers of these species such as Ceratium furca have been known to damage fish gills and to create anoxic conditions by depleting dissolved oxygen from the environment, which can suffocate various animals in the area [39].

As a whole, the highest presence of rare taxa shows of a permanent renewal of taxa belonging to the genera researched which could be very sensitive to the variations of environment conditions. The specific composition and the dynamics of the populations algales during this study would be influenced by innumerable physico-chemical factors and by the biological interactions like chattering by the zooplancton and the competition inter and intraspecific.

The spatial and seasonal distribution of species in the section studied showed overall dominance of the genera Protoperidinium. The number of species identified for this genera was greater than the number for Ceratium. Some species of this genera may survive for a longer period of time, for example, Protoperidinium depressum may survive up to 71 days in states of starvation or low food availability [40]. Smaller to larger diatoms and dinoflagellates are the primary food source for Protoperidinium, and their proliferation can cause a red tide of Protoperidinium [41].

Unlike the diversity that was dominated by the genera Protoperidinium, the densities were dominated by the species belonging to the genera Ceratium at all collection points and in all seasons. This dominance in terms of density could be explained according to Dodge and Marshal [13] who emphasizes that genera Ceratium is an omnipresent kind of dinoflagellates to theca, slow growing, found during all seasons and contributes significantly to the annual primary production in the world’s oceans. In particular, this density dominated by the species Ceratium furca and Ceratium fusus. On the coast of Kribi, the appearance of a bloom of one of these two species could be predicted during the short and long rainy season at points P2, P5, P6 and P7. They have more preferences according to the Factor Analysis of the Correspondences at points P2 and P6. Their strong presence could be justified by the fact that Ceratium furca is a species commonly found in coastal waters and whose ecological impact caused by flowers has intensified in recent years [42] while Ceratium fusus is classified as a cosmopolitan species found in an ambient temperature between 2˚C and 29.5˚C [13]. The positive and significant correlation recorded testifies to the importance of temperature for the development of this species.

Despite their low density observed during this study, some species belonging to the genera Protopéridinium should be monitored. Protoperidinium divergens, Protoperidinium steinii, Protoperidinium crassipes identified during this work have been recognized as harmful by several works. Positive and significant correlations between Protoperidinium divergens, temperature and orthophosphates; Negative and significant between Protoperidinium divergens and dissolved oxygen observed show that an increase in temperature, an enrichment in orthophosphate and a decrease in dissolved oxygen, could cause the efflorescence of this species in the coast of Kribi. The temperature seems to be favorable for the flower of this genera, as reported by Raji and Padmavati [18] during the flower of Protoperidinium divergens in the Andaman compartment region. In addition, phosphorus is an essential nutrient that causes eutrophication leading to lush algae growth when their concentrations exceed certain limits [43] [44]. Overall, the majority of Protoperidinium species appear to be diatom grazers, some may use, or even require dinoflagellate prey for their proliferation while others may feed on prey larger than themselves [44].

In reference to similar studies carried out in the world [18] [39] [42] [45], this work has allowed to count in the study area 06 species of the studied genera which could cause in time phenomena such as the appearence of algal blooms and fish poisoning never experienced in the Cameroonian coast. These are Ceratium furca, Ceratium fusus, Ceratium tripos, Protoperidinium divergens, Protoperidinium steinii, Protoperidinium crassipes.