Temporal variation in the phytoplankton community of Acapulco Bay , Mexico

Species composition and abundance of the phytoplankton community in Acapulco Bay, Mexico, were studied from May to December 2009. Samples were collected at 5 stations (4 coastal and one oceanic) at 3 depths (1, 5 and 10 m). Eighty-seven species were identified: 54 dinoflagellates, 32 diatoms and one silicoflagellate. The community was structured mainly by adiaphoric species, that is species adapted to both neritic and oceanic environments. Species abundance and composition varied significantly during the sampling period due to increased nutrient concentrations in the rainy season. Dinoflagellate species were more abundant during the dry season, and diatom species dominated numerically during the rainy season.


Introduction
Phytoplankton is one of the most complex communities in marine coastal environments.This community's structure is dictated by two important groups of organisms: i) non-motile, fast-growing diatoms; and ii) motile flagellates and dinoflagellates which can migrate vertically in the water column in response to light.All phytoplankton species are subject to water currents and have developed strategies for rapid nutrient absorption and fast reactions to fluctuations in hydrographic conditions. 1 Thus, phytoplankton distribution and species composition are affected by several processes, including high water temperature, and variations in thermal stability and nutrient circulation.
Changes can occur in the taxonomic composition of phytoplankton communities, the total cell abundance and species richness during annual seasonal cycles.These changes reflect the capacity of communities to respond to seasonal variations in light, nutrient and circula-tion patterns. 2Under specific environmental conditions (e.g.high nutrient concentrations) these factors can cause some species to proliferate massively, causing harmful algal blooms (HAB) which can negatively affect marine communities.][5] Santa Lucia Bay, also known as Acapulco Bay, is located on the tropical Pacific coast of southern Mexico, in Guerrero State.The bay has a semi-circular (6.3 km diameter), amphitheater-like shape created by low hills (<500 m) surrounding the south-facing bay.It is considered to be very climatologically protected (Agustín, unpublished data, 2011).
Research on marine phytoplankton communities abounds, [6][7][8][9] but to date there have been no studies assessing the phytoplankton species composition in Acapulco Bay, Mexico.The aim of the present study was to document current species composition and abundance in the phytoplankton community of Acapulco Bay, and quantify any temporal variation in these variables.
Nutrient (nitrates, ammonium and phosphates) concentrations were determined in each sample following a standard colorimetric method (Hanna equip).Phytoplankton samples were fixed in concentrated Lugol's solution and cell quantification was made using the Utermöhl chamber sedimentation concentration method. 102][13][14][15][16] Phytoplankton species distribution was established based on a review of published records for Mexico and other countries, and each species was classified as: (ES) estuarine, (NE) neritic, (AD) adiaphoric or (OC) oceanic.
The Olmstead-Tukey association test 17 was applied to classify the phytoplankton species based on parameters of occurrence frequency and mean abundance: (D) dominant (abundant and frequent); (C) common (low abundance but frequent); (O) occasional (abundant but low frequency); and (R) rare (low abundance and low frequency).A one-way ANOVA was applied to identify significant differences in monthly parameters (temperature, pH and salinity) and nutrient concentrations.When significant deviations from normality were identified, parameters and concentration data were log-transformed (log ¥+1) to meet normality and homoscedasticity requirements.Any differences in phytoplankton abundance at the 3 tested depths were identified with a oneway ANOVA.A χ 2 test was applied to identify significant differences in mean phytoplankton group abundance between months.
Community parameters included total number of species, total number of cells, the Shannon-Wiener (H) diversity index, species evenness (J) and the Berger-Parker Index (BPI) as a measure of numerical dominance. 18,19The qualitative Jaccard similarity index was used to evaluate similarity or difference in species composition between months.Student's t-test was applied to identify differences between community parameters, and correlations calculated using the Spearman's range coefficient (r s ) were applied to identify the parameters that determined observed species distribution and composition patterns.
Phytoplankton community taxonomic composition included a total of 87 species: 54 dinoflagellates (Dinophyta), 32 diatoms (Bacillariophyta) and one silicoflagellate (Heterokontophyta).Although the number of microalgal diatom species was relatively low, this group's total cell abundance represented more than 70% of the total cells collected during the sampling period.Phytoplankton species group differentiation showed that dinoflagellates dominated numerically from May to June, while diatoms increased constantly in abundance beginning in July and peaked between September and December (Figure 3).Monthly mean diatom species abundance was positively correlated with nitrate (r s =0.857, P<0.05) and phosphate (r s =0.786, P<0.05) concentrations; dinoflagellate abundance exhibited no correlation.
Six species numerically dominated the community: 2 dinoflagellate species (Neoceratium belechii and Scrippsiella trochoidea) and 4 diatoms (Chaetoceros tetrastichon, Leptocylindrus danicus, Pseudosolenia calcaravis, and Pseudonitzschia sp.) (Table 1).These six species accounted for 66% of the total estimated cell counts in the samples.The dinoflagellates Dinophysis caudate, Neoceratium furca and Protoperidinium divergens were identified in all samples though none were numerically dominate.Based on phytoplankton species distribution, over 70% of the dinoflagellate and diatom species were classified as adiaphoric (i.e. they occur in neritic and oceanic areas) (Figure 4).The frequencyabundance graphic method indicated that 60% of the diatom species were dominant (i.e.abundant and frequent) while only 27% of dinoflagellate species were classified as dominant (Figure 5).
No differences in phytoplankton abundance (cell/L) were observed at the 3 sampled depths (1, 5 and 10 m) (ANOVA F=0.283, P>0.05), therefore all abundance data were pooled for subsequent analyses.Monthly mean abundance varied (χ 2 test, P<0.05) between sam-pling stations, with stations 1 (Club de Yates) and 4 (Casa de Diaz Ordaz) each having the highest mean values for three months, although not in the same months (Figure 6).Mean abundance was highest at station 1 from September to November, but highest at station 4 in May, June and September (Figure 6).

Discussion
Water temperature was almost constant from June to December, varying only 3°C most months (Figure 1), whereas salinity decreased significantly during the same period (Figure 1).Temperature fluctuations are not as extreme in tropical marine areas as those seen in temperate or cold latitudes, suggesting that this environmental parameter is not very informative in terms of species composition in tropical phytoplankton communities. 9The fluctuations in salinity values that were observed can be attributed to freshwater flowing into the bay during the rainy season (June to November).Salinity is known to have a notable effect on the structuring of phytoplankton communities, particularly in estuarine and coastal environments. 8,9,20,21he constant increase in nitrate and phosphate concentrations between June and November can also be attributed to rainfall runoff during this season. 7Sewage, as well as huge quantities of garbage and other solid materials from the Acapulco urban area discharge directly into the bay, contributing to higher nutrient concentrations and consequent population growth in several phytoplankton species. 7,9he dinoflagellates (54 species) dominated the phytoplankton community in Acapulco Bay (Table 2), although total abundance of the group was significantly lower than that of the diatoms.9]22 Phytoplankton are considered to be good indicators of water conditions in neritic and oceanic masses. 23,24Dinoflagellates are better adapted to the oceanic environment, while diatoms are more adapted to coastal environments. 8,23Therefore, our results suggest that environmental conditions in Acapulco Bay change during the year in response to variations in several environmental parameters.Changes in the bay's water masses are probably reflected in the traits presented by the phytoplankton species inhabiting it; for example, more than 70% of the dinoflagellate and diatom species recorded in the present study are adapted to neritic or oceanic environments, that is they are classified as adiaphoric species. 8(Figure 4).

Article
The pattern of species distribution of the phytoplankton community in Acapulco Bay was similar to patterns reported for other marine or estuarine communities.Community structure was determined by a low number of dominant species (2 dinoflagellates and 4 diatoms) which contributed over 60% of total abundance, and a high number of occasional and rare species (>50%) (Figure 5).Total richness (87 species) in the study area was within the species number range reported for other locations in Mexico 26 and other countries with similar climatological conditions. 7,8,22The diversity values recorded here are similar to those reported for other locations in the tropical Pacific. 8The higher diversity values recorded in the months of July (3.2) and October (3.23) can be attributed to the fact that some species' dominance diminished, whereas in other months these species' abundances (and therefore their dominance) remained fairly constant (evenness ≥0.60,Table 1).The low similarity values observed here (Figure 7) suggest the possibility that species composition differed between months in response to variability in environmental conditions over time.
Overall, the results indicate that phytoplankton species composition and abundance in Acapulco Bay varied spatially and temporally due to variations in environmental conditions.These environmental variations were caused by changes in nutrient concentrations which were a result of differences in rainfall between the local dry and rainy seasons.Higher nutrient concentrations promoted population growth in dinoflagellate or diatom species at different time points during the year.

Figure 1 .
Figure 1.Monthly temperature and salinity values in Acapulco Bay, Mexico, between May-December.

Figure 3 .
Figure 3. Temporal variation of dinoflagellate and diatom groups in Acapulco Bay, Mexico.

Figure 7 .
Figure 7. Similarity percentages for monthly phytoplankton samples in Acapulco Bay, Mexico.
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