Ecological guilds of epiphytic diatoms (Bacillariophyta) on Acrostichum danaeifolium Längst. & Fisch in a subtropical wetland in southern Brazil

: Aim: Seasonal patterns diatom community on Acrostichum danaeifolium were examined in a wetland in southern Brazil. Methods: The adhered diatoms were removed from the plant, species identification and growth forms were performed, and determined physical-chemical parameters of water. Results: In total, 96 taxa belonging to 46 genera were identified. Nitzschia frustulum , Pseudostaurosira brevistriata and Plagiogramma tenuissimum were abundant species. Distinct growth forms that formed low-and high-profile ecological guilds and a mobile guild were observed. Navicula and Nitzschia were the genera with the greatest number of species, and these diatoms formed mucilage tubes. The water temperature varied from 10-26 °C, the depth from 0.35-0.80 m, the transparency from 0.20-0.23 m, the flow from 9.4-42.12 m 3 s –1 , the pH from 7.08-8.89, the electrical conductivity from 0.65-15.83 mS cm –1 , the total organic phosphorus from 0.03-0.11 mg L –1 , and the total organic nitrogen from 0.29-0.49 mg L –1 . In summer, marine species such as Thalassiosira eccentrica and Rhaphoneis castracanii were also present. Conclusions: The high-profile guild prevailed in all seasons of the year, with higher number of growth forms in the guild in winter. The richness found on A . danaeifolium shows that this plant provide a favorable habitat for epiphytic diatoms in wetlands such as Lagoa Pequena.


Study area
The Pseudônimo stream (31°40 '16,4" S and 52°04'51,4" O) is a water course connected to the Patos Lagoon estuary and is associated with the spillway of Lagoa Pequena, a Brazilian subtropical wetland located in the coastal plain of Rio Grande do Sul (Figure 1).The dominant vegetation is a Cyperaceae Schoenoplectus americanus (Pers.)Volk ex Schinz & Kell.

Sample collection and chemical and physical analysis
Twelve samples were collected seasonally (fall (May), winter (August), spring (December) and summer (January)) during the period from May 2011 to January 2012.
The samples were fixed with 4% formalin.The adhered diatoms were removed with the aid of a 10 mm brush and distilled water jets.Each sample contained 20 ml, and 2 ml was removed for the preparation of the permanent slides with Naphrax  resin, according to the methods of Simonsen (1974).Twenty-four permanent slides were analyzed, which included two slides per sample.
To study the ecological guilds, one sample (not fixed) was collected at each station to facilitate observations of the diatom growth forms on simple slides (slide and cover glass).The slides were observed with an optic microscope Olympus BX 40.The diatoms were grouped according to the growth morphology of the three guild types according to Passy (2007).

Analysis of the diatom community
At least 400 valves of diatoms were counted per sample, with counts of approximately 200 valves per permanent slide.Relative abundance was calculated based on the number of valves counted and the number of species found on each slide.Species with values higher than the average calculated for each slide were considered abundant (Lobo & Leighton,

Introduction
South America has large expanses of wetlands, and most of them (50%) are located in Brazil.Because wetlands comprise a large number of natural environments, ecological studies of these areas are highlighted as important tasks in limnology (Neiff, 2001;Naranjo, 1995;Esteves, 2011).In the State of Rio Grande do Sul, the Lagoa Pequena is a wetland designated as a conservation priority (Base de dados tropical, 2003).
The vast majority of studies of epiphytic diatoms are focused on macroalgae and the under growth of marine vegetation (Costa et al., 2009).In Brazil, there are records of diatoms in angiosperms, such as Polygonum hydropiperoides Michaux.and Potamogeton polygonus Cham.& Schltdl (Bertolli et al., 2010;Santos et al., 2011), and in China there are records of epiphytic diatoms on the angiosperm Kandelia candel L., (Chen et al., 2010).This work focused on the emergent aquatic macrophyte A. danaeifolium because this plant supports the colonization and development of adhered diatoms.There are no records of a study on the relation between epiphytic diatoms and this plant.The study of ecological guilds may reveal the potential of species to use resources and avoid system disturbances (Passy, 2007) while also indicating the complexity of the community through the presence of distinct guilds.
In this context, the objective of this work was to determine the specific composition of epiphytic diatom community on A. danaeifolium and the ecological guilds that are present, as well as to determine which environmental factors influence the presence of these guilds.2015, 27(3), 311-321 Ecological guilds of epiphytic diatoms (Bacillariophyta)…

General characteristics of the diatom community
In total, 96 species of diatoms distributed in 46 genera were identified from the 12 samples examined.Most of the species found were from freshwater and brackish environments and have a cosmopolitan distribution.The two genera that showed the greatest species richness were Nitzschia (19 spp) and Navicula (6 spp) (Figure 2).Species richness was 29 (winter), 72 (autumn), 45 (spring) and 64 (summer).
1986).The calculated parameters include diversity index (Shannon), equitability and dominance, and these calculations were performed with the software PAST (Hammer et al., 2001).Also, to compare the specific diversity among the seasons, the "t" test was used (α = 0.05).
The occurrence of Navicula and Nitzschia as epiphytes is rare; the most common occurrence was of Cocconeis in marine environments (Chen et al., 2010) and Gomphonema in freshwater environments (Tremarin et al., 2009).
Acrostichum danaeifolium showed restricted distribution in Lagoa Pequena, occurring only in a fragment of 500 meters along the western margin of the Pseudônimo Stream (Figure 3).
Through the study of live samples (simple slides), the ecological guild was determined to comprise 21 species.In addition, a high density of epiphytic diatoms with different growth forms (erect, adnate, chain, with mucilage tube, stalked (peduncle of mucilage) and mobile), were part of the low-profile, high-profile and motile guilds (Table 1).
A significant difference in diversity was observed in the different seasons (p = 0.001).Greater diversity and numbers of species occurred in autumn and summer; and lower in winter and spring (Table 2).Greater numbers of abundant species occurred in autumn (9) and summer ( 16); and lower numbers occurred in winter (5) and spring (7) (Table 1).
The temperature can have a significant effect on the growth and mortality rates of epiphytes (Borowitzka et al., 2006) and can be the main parameter related to the species richness and abundance both in winter and in summer.In winter, with a decrease of 8 °C, richness decreases by approximately 60% by decreasing the number of abundant species from nine to five.In summer, with an increase of 14 °C (relative to the winter) species richness increases to approximately 56% and the numbers of the abundant species also increases.
In ecological terms, algae epiphytes are an important food source for invertebrates living in the Patos Lagoon estuary, such as the polychaete Laeonereis acuta (Benvenuti & Colling, 2010), and  *Indicates especies not recorded as an abundant epiphyte in other estuaries and wetlands of the American continents.*Seasons with significative difference in diversity (α=0.05).Pseudostaurosira brevistriata) met the definition for a high-profile guild for this season of the year.The mucilage tubes were formed by the genera Nitzschia, Navicula and Ctenophora (this last one was observed only once in the winter).Three species occurred in the spring, four in summer and seven in autumn.In spring and summer (during lower water flow), a greater number of species were expected to form the high-profile guild because, according to Passy (2007), the lower flow favors this profile.However, this was not observed, and the largest species number observed in the high-profile guild occurred in greater flow (winter).The sorting of the samples according the physical and chemical variables (Table 3) and the composition of species as descriptors revealed the separation of the samples by the seasons of the year (autumn, winter, spring and summer) in two main groups (Figure 5).
In the summer, a high number of mobile species (6spp) were present; however, the presence of chain-forming species (Bacillaria paxilifera and

Epiphytic diatoms on different substrates
According to Passy (2007), Navicula and Nitzschia are part of the mobile guild, but in A. danaeifolium, Navicula spp.and Nitzchia spp.were found to be mobile and formed mucilage tubes, thus representing both the high-profile guild and the mobile guild.

Seasonal change in diatom community
The low-profile guild was represented by Cocconeis placentula and Gomphonema parvulum; the latter was assigned to this guild for presenting short peduncles, according to Passy (2007).Cocconeis placentula was abundant in the spring and summer, when there was a reduction in the number of mucilage tube-forming species, which favored its abundance.
The first group (summer and autumn) showed a positive correlation with temperature, conductivity and total phosphorus and a negative correlation with the depth, transparency, total nitrogen and pH.This group was defined as predominant in the Pseudônimo Stream.
The second group (winter) showed a positive correlation with water transparency, depth, total nitrogen and pH, and a negative correlation with electrical conductivity, temperature and total phosphorus.
The values of the nutrients observed during the study period in Pseudônimo Stream allowed the stream to be considered of good quality because the phosphorus was within the limits established by the National Council on the Environment (Brasil, 2005) for brackish waters of class 1.The sorting of samples also showed that some species of the high-profile guild (chain-forming, tubesand erect) responded (with greater abundance) to environmental factors such as temperature in summer (Pseudostaurosira brevistriata), total phosphorus in autumn (N.subchoaerens var.scotica, N. palea and N. clausii) and water transparency in winter (Navicula sp. and Ctenophora pulchella) (Figure 5).

Figure 1 .
Figure 1.Location of Pseudônimo Stream in southern Brazil.
, the pink shrimp Farfantepenaeus paulensis and the blue crab Calinectes sapidus (D'Incao & Dumont, 2010).Thus, epiphytic diatoms of the Pseudônimo Stream can contribute to the trophic relations and productivity of the Lagoa dos Patos estuary.

Figure 2 .
Figure 2. Relative distribution of the genus that accounts for at least three species of epiphytic diatoms in A. danaeifolium the period of May 2011 to January 2012 (total number of valves counted).

Figure 3 .
Figure 3. Acrostichum danaeifolium.(a) General view of the fragment of vegetation along the western margin of the Pseudônimo Stream.(b) Detail of the baculum.(c and d) Vegetation during the normal level (winter) and in (d), detail of the periphyton adhered to the pinnae.(e and f ) Period of low water level (summer) with few fronds in contact with water, and in (f ) detail of the pinnae (the part collected for observation of the diatoms).

Figure 4 .
Figure 4. Relative distribution of the growth forms and type of guild (HP = high profile, LP = low profile and MG = mobile guild) in A. danaeifolium in the Pseudônimo Stream in the period of May 2011 to January 2012.

Table 1 .
List of taxa that were used in the canonical correspondence analysis (with abundance ˃1% and acronyms) including 25 abundant species (bold), and 21 growth forms.

Table 2 .
Seasonal changes in the richness, dominance, equitability, and diversity of the diatom community in A. danaeifolium.