O papel dos recursos regulando a dominância de gêneros de cianobactérias em reservatórios do semiárido tropical

UNIVERSIDADE FEDERAL DO RIO GRANDE DO NORTE PROGRAMA DE PÓS-GRADUAÇÃO EM ECOLOGIA

O PAPEL DOS FATORES AMBIENTAIS REGULANDO A

DOMI-NÂNCIA DE GÊNEROS DE CIANOBACTÉRIAS EM

RESERVA-TÓRIOS DO SEMIÁRIDO TROPICAL

RAYANE

FERNANDES

VANDERLEY

N

,

8

THE

ROLE

OF

ENVIRONMENTAL

FACTORS

DRIVING

THE

DOMINANCE

OF

CYANOBACTERIA

GENERA

IN

TROPICAL

SEMIARID

RESERVOIRS

RAYANE

FERNANDES

VANDERLEY

Dissertação apresentada ao Programa de

Pós-Graduação em Ecologia, da Universidade Federal do

Rio Grande do Norte, como parte dos requisitos para

obtenção do título de Mestre em Ecologia.

Orientadora: Profª.Renata Panosso Co-Orientadora: Dr.

Kemal Ali Ger

Comissão Examinadora Profª. Dra. Juliana Dias

Profª. Dr. Rosemberg Menezes Profª. Dra. Vanessa Becker

9 Universidade Federal do Rio Grande do Norte - UFRN

Sistema de Bibliotecas - SISBI

Catalogação de Publicação na Fonte. UFRN - Biblioteca Central Zila Mamede

Vanderley, Rayane Fernandes.

O Papel dos recursos regulando a dominância de gêneros de ci-anobactérias em reservatórios do semiárido tropical / Rayane Fer-nandes Vanderley. - 2019.

34 f.: il.

Dissertação (mestrado) - Universidade Federal do Rio Grande do Norte, Centro de Biociências, Programa de Ecologia, Natal, RN, 2019.

Orientadora: Profa. Dra. Renata Panosso. Coorientador: Prof. Dr. Kemal Ali Ger.

1. Floração da cianobactéria - TCC. 2. Seca - TCC. 3. Cylin-drospermopsis raciborskii - TCC. 4. Microcystis aeruginosa - TCC. 5. Eutrofização - TCC. I. Panosso, Renata. II. Ger, Kemal Ali. III. Título.

RN/UF/BCZM CDU 561.232 Elaborado por Ana Cristina Cavalcanti Tinôco - CRB-15/262

10

If you have a dream, fight for it. There is a dis-cipline for passion. And it is not about how many times you get rejected or you fall down or you are beaten up. It is about how many times you stand up and are brave and you keep on going.

11

A

Aos meus pais, pelo dom da vida e por terem me permitido chegar até aqui;

Aos meus tios, Fátima e Franklin Capistrano por terem me adotado no momento que

mais precisei;

A minha irmã, por ser essa pessoa tão maravilhosa e por ter enxugado meu choro

tan-tas vezes;

Ao meu co-orientador Ali, que me ensinou não apenas a ser uma cientista melhor, mas

também uma pessoa melhor, me ajudando a enfrentar meus medos e me puxando aos

meus limites, por isso serei eternamente grata;

A minha orientadora Renata, por ter me acolhido desde graduação e por ser sempre

tão compreensível e ter me guiado em momentos difíceis;

A Vanessa Becker pelos ensinamentos científicos, paciência com minhas dúvidas e

pa-lavras de apoio;

Ao professor José Attayde por ter viabilizado o projeto;

Ao pessoal do IGARN que subsidiou as coletas;

Aninha, Gabi, Gabriella, Walter e Julie por terem me ajudado com as análises do

pro-jeto, sem vocês não teria conseguido;

A todos meus amigos do LAMAQ que sempre foram tão solícitos e disponíveis para me

ajudar em tudo que precisei;

A Bruna por tamanha paciência em me ensinar estatística e por me salvar a cada vez

que o R dava problema;

Aos meus amigos, por entenderem minha ausência, por terem sempre ficado ao meu

lado e por tantas palavras de carinho e apoio;

12

O PAPEL DOS FATORES AMBIENTAIS REGULANDO A

DOMI-NÂNCIA DE GÊNEROS DE CIANOBACTÉRIAS EM

RESERVA-TÓRIOS DO SEMIÁRIDO TROPICAL

RAYANE FERNANDES¹ KEMAL ALI GER2 VANESSA BECKER3 MARIA GABRIELA TRIGUEI-RO3 RENATA PANOSSO¹

¹Programa de Pós-graduação em Ecologia, Universidade Federal do Rio Grande do Norte, Natal, RN, Brasil 2

Universidade Federal do Rio Grande do Sul, Campus Litoral Norte, Tramandaí, RS, Brasil. 3

Departamento de Engenharia Civil, Universidade Federal do Rio Grande do Norte, Natal, RN, Brasil

RESUMO

Florações nocivas de cianobactérias são consideradas um dos maiores problemas em ambientes de água doce. Recentemente, sua frequência e duração vêm aumentando. Esse é o cenário da grande parte dos reservatórios localizados no semiárido do Brasil, nos quais apresentam flora-ções persistentes de cianobactérias. Os fatores regulando essas floraflora-ções persistentes de cia-nobactérias permanecem pouco compreendidos, e sua compreensão é essencial para o desenvol-vimento de estratégias que visem melhorar a qualidade da água. À vista disso, o objetivo do presente estudo foi elucidar os fatores regulatórios da dominância de cianobactérias; avaliar os efeitos da seca na disponibilidade de recursos e no fitoplâncton; e diferenciar preferências am-bientais das espécies Cylindrospermopsis raciborskii e Microcystis aeruginosa. Parâmetros químicos, físicos e biológicos de seis reservatórios localizados na região semiárida do Brasil foram analisados. “Cylindropermopsis abundância index” foi criado para acessar as preferên-cias ambientais das duas espécies mais representativas. Nossos resultados indicam que reserva-tórios eutróficos do semiárido são mais suscetíveis a florações permanentes de cianobactéria. Nitrogênio e fósforo favorecem a dominância de Cylindrospermopsis e Microcystis, entretanto luz aparenta ser o fator que regula a alternância de dominância entre essas espécies. Seca influ-enciou negativamente a concentração de fósforo total e biomassa do fitoplâncton. Em suma, eutrofização em reservatórios do semiárido pode conduzir a floração persistente de cianobacté-ria, porém, outros fatores, como físicos, regulariam a mudança temporal de espécies. Além dis-so, seca potencializaria a dominância persistente de cianobactérias, pois interfere com a dispo-nibilidade de recursos.

Palavras chaves: Seca, Floração persistente de cianobactéria, Cylindrospermopsis raciborskii, Microcystis aeruginosa, eutrofização.

13

THE

ROLE

OF

ENVIRONMENTAL

FACTORS

DRIVING

THE

DOMINANCE

OF

CYANOBACTERIA

GENERA

IN

TROPICAL

SEMIARID

RESERVOIRS

RAYANE FERNANDES¹ KEMAL ALI GER2 VANESSA BECKER3 MARIA GABRIELA TRIGUEI-RO3 RENATA PANOSSO¹

¹PROGRAMA DE PÓS-GRADUAÇÃO EM ECOLOGIA,FEDERAL UNIVERSITY OF RIO GRANDE DO NORTE,NATAL,RN, BRAZIL. 2

FEDERAL UNIVERSITY OF RIO GRANDE DO SUL -CAMPUS LITORAL NORTE,RIO GRANDE DO SUL,RS,BRAZIL. 3

DEPARTAMENT OF CIVIL ENGINEERING,FEDERAL UNIVERSITY OF RIO GRANDE DO NORTE,NATAL,RN,BRAZIL

ABSTRACT

Harmful cyanobacterial blooms have been long recognized as one of the most severe problems in freshwater systems. Recently, their frequency and duration have been increasing. This is the scenario for many semiarid reservoirs in Brazil that face persistent cyanobacterial blooms. The drivers regulating persistent cyanobacterial blooms are still poorly understood and compre-hend them is crucial to the development of strategies to improve water quality. In light of this, we aim to elucidate the drivers of cyanobacteria dominance; evaluate the effect of drought and differ environmental preferences of Cylindrospermopsis raciborskii and Microcystis

aeruginosa. We analysed chemical, physical and biological parameters from six reservoirs

located in the semi-arid region from Brazil and created a “Cylinder abudance index” to access environmental preferences of the two most representatives species. Eutrophic semiarid reser-voirs are more susceptible to persistent cyanobacteria bloom. Nitrogen and phosphorus favoured Cylindrospermopsis and Microcystis dominance, but light seems to be the factor regulating the switch between them. Drought negatively influenced total phosphorous and phytoplankton bio-mass. Our results suggest eutrophication in semiarid environments may lead to persistent domi-nance of cyanobacteria, still, other factors as physical could drive the temporal change of specie. Drought can potencialize persistent dominance due to interfere in the availability of resources.

Keywords: Drought, persistent cyanobacteria dominance, Cylindrospermopsis raciborskii, Microcystis aeruginosa, eutrophication.

14

Summary

INTRODUCTION ... 15

MATERIAL & METHODS ... 17

STUDY SITE ... 17

SAMPLING AND ANALYSIS ... 18

STATISTICAL ANALYSES ... 19

RESULTS ... 20

3.1LIMNOLOGICAL SCENARIO ... 20

3.2PHYTOPLANKTON COMMUNITY AND CYANOBACTERIA DOMINANCE ... 20

3.3RELATIONSHIP BETWEEN CYANOBACTERIA DOMINANCE AND EXPLANATORY VARIABLES ... 23

3.4CYLINDROSPERMOPSIS AND MICROCYSTIS ENVIRONMENTAL PREFERENCES ... 25

DISCUSSION ... 26

15

I

The link between human nutrient over-enrichment, eutrophication and harmful cyanobacterial blooms has been long recognized as a critical threat to freshwater systems. Moreover, some cyanobacteria genera are key indicators of decreased water quality, through generate conditions of hypoxia and disrupt energy flux in the aquatic food webs, leading to loss of biodiversity (Paerl & Otten, 2013). Also, the production of toxins poses a potential hazard for health of wild-life and humans, especially when drinking water supplies are affected. Besides this, eutrophica-tion interfere in many ecosystem services, promoting significant economic losses, including increasing water treatment for drinking supply, decline of commercial and subsistence fishing and recreation (Aylward et al., 2005; EPA, 2015)

This is the scenario of many shallow reservoirs in Brazilian tropical semiarid region, which can serve for multiple purposes, and are vulnerable to nutrient input from anthropic sources, especially diffuse ones. Still, internal load also plays an important role. Semi-arid res-ervoirs are susceptible to phosphorus release from sediments depending on environmental con-ditions, such as low depths, wind resuspension of particles, and anoxia in the hypolimnion (Cavalcante et al., 2018).

Brazil’s semi-arid region has a highly seasonal rainfall concentration, high solar irradi-ance and low amplitude of seasonal temperature, along with severe periods of drought. Alt-hough, the effect of drought in semiarid reservoirs has been recently investigated its effects still remain unclear. Drought could either favour phytoplankton growth or induce growth suppres-sion due to high inorganic turbidity depending of reservoir (Braga et al., 2015), collapse total phytoplankton biomass (Figueiredo & Becker, 2018), magnify eutrophication and cyanobacte-ria dominance (Brasil et al., 2016; Rocha Junior et al., 2018). On the other hand, evidences im-plicate that severe drough leads to reduction of resources and cyanobacteria biomass favouring potential mixotrophic organisms (Castro Medeiros et al., 2015; Costa et al., 2019). Hence, in all situations water quality decline dramatically.

Identifying the local drivers structuring cyanobacteria assemblage has been highlighted as an important issue of classical and contemporary limnological research, since it is a crucial step to successful water management, especially in semi-arid regions, where most of the reser-voirs face persistent dominance of cyanobacteria (Moura et al., 2018). Cyanobacteria domi-nance is regulated by the combined, and often synergetic, effects of abiotic factors, as nutrient inputs, light, temperature, water residence/flushing times, and biotic interactions, as competi-tion, allelopathy and grazing (Paerl, 2017). Furthermore, external factors, such as rainfall and flushing, can also play an important role in cyanobacteria dynamics because they interact with

16 water parameters (Havens et al., 2019). Nowadays, with global warming scenario the positive relation between temperature and cyanobacterial dominance has been in the spotlight (Paerl & Huisman, 2008; O’Neil et al., 2012; Bonilla et al., 2016; Paerl, 2017; Rollwagen-bollens et al., 2018), at least for temperate regions.

In tropical waters, however, with high input of nutrients and constant light incidence, the regulators of persistent cyanobacterial blooms still poorly understood. Several studies provided different insights about cyanobacteria dominance. For example, temperature, mixing zone and light intensity played a major role in regulating cyanobacterial dominance (Soares et al., 2009). Meanwhile, long-lasting blooms were positively related to water column stability, pH, and total nitrogen (Brasil et al., 2016), strongly influenced by depth and flushing (Havens et al., 2019) and driven by inorganic turbidity (da Costa et al., 2016; González-Madina et al., 2019).

Regarding chemical environment, since Tilman and Kilham (1982) many authors have considered the ratio of limiting nutrients, nitrogen to phosphorus (N/P), the major determinants of phytoplankton community structure in freshwater ecosystems (Havens et al., 2003; Nõges et al., 2008; Vrede et al., 2009), specially to cyanobacteria. Low N:P ratio leads to N-limitation of primary producers; in this case, N-fixing cyanobacteria, would be favoured (Smith, 1983). Nev-ertheless, total N and P have been pointed out to better predictors of species composition (Downing et al., 2001). Nutrient concentrations along with light intensity have been long recog-nized as vital resources for phytoplankton growth (Tilman, 1982).

In reservoirs facing constant dominance of cyanobacteria, it is essential to understand the factors behind the temporal changes in cyanobacteria genera dominance, considering that some of them demand even more careful actions to improve water quality. Although cyanobac-teria share certain key traits, they are a heterogeneous group, in which each genus possesses different ecophysiological adaptations and benefits from different environmental conditions (Mantzouki et al., 2016). Furthermore, cyanobacteria would vary in their response to changes in the availability of resources depending on morphological and physiological traits.

In this article, we explore some factors that may drive the temporal variability of cyano-bacteria dominance in Brazilian tropical semi-arid reservoirs, this could provide support to the development of successful strategies to controlling harmful blooms. The aim of the present study was to (1) elucidate the main abiotic drivers regulating the dominance of different bacteria genera; (2) evaluate the influence of drought on concentration of nutrients and cyano-bacteria dominance and (3) differ environmental preferences of Cylindrospermopsis raciborskii and Microcystis aeruginosa.

17

MATERIAL

&

S

We studied six reservoirs located in the semi-arid region of Brazil (Table 1) during the year of 2017. The climate of this region has an average temperature higher than 25°C, elevated evapo-ration rates and irregular rainfall concentrated in only a few months of the year. The accumulat-ed precipitation for this region was 567.1 mm.year-1 in 2017 (SEMARH, 2017). All reservoirs had their volume below their maximum capacity during the study due to severe drought in the region that began in 2012 (CEMADEN, 2019) .The morphometric variables, area and maxi-mum capacity of the reservoirs, were obtained from the State Department of the Environment and Water Resources (SEMARH) (Table 2). The studied reservoirs studied are used for multiple proposes, including irrigation, livestock and human drinking water.

Table 1 Geographic coordinates of each reservoir studied.

Reservoirs Latitude Longitude

Encanto -006° 07' 211408 -038° 19' 04.586 Boqueirão Parelhas -006° 41' 42.2670 -036° 37' 43.510 Tabatinga -005° 56' 21.1210 -035° 24' 44.269 Prata -006° 14' 45.6737 -035° 16' 31.665 Pajuçara -006° 09' 18.1627 -035° 25' 21.114 Santa Cruz do Apodi -005° 45' 58.3821 -037° 47' 56.857

Table 2 Morphometric variables of the six reservoirs included in the study. Pajuçara’s reservoir data for area and maximum capacity were not available (NA). Data source: State Department of the Environment and Water Re-sources, (SEMARH).

Reservoir Basin Area (ha)

Maximum Capacity (m3)

Boqueirão Parelhas Piranhas/Assu 1.267,27 84.792.119

Encanto Apodi/Mossoró 123,78 5.192.538

Prata Jacú 151,00 9.321.149

Tabatinga Potengi 1.090,00 89.835.678 Santa Cruz do Apodi Apodi/Mossoró 3.413,36 599.712.000

18

S

Monthly samplings from January to December of 2017 were carried out. Water samples were collected with a PVC tube from the two first meters near to surface; this procedure was repeated five times spaced around the collecting point, close to the dam wall in the deepest area of the reservoir. Water samples from each point were integrated and kept into pre washed bottles to posterior analyses, for chlorophyll-a (400 mL), total phosphorous and nitrogen (200 mL), and phytoplankton (200 mL). Phytoplankton samples were fixed with acetic Lugol solution. Water temperature was measured in situ at the bottom and top of water column.

Water transparency and maximum depth (Zmax) were accessed by a Secchi disk, then and the euphotic depth (Zeu) was calculated as 2.7 times the Secchi measure (Cole, 1994). Total nitrogen (TN) and dissolved nitrogen (TDN) were analysed by standard techniques with the SHIMADZU TOCVCPN sampler with the SSM-5000A solid sample combustion unit by

chemi-luminescence. Total phosphorous (TP) and dissolved phosphorous (TDP) were determined through the persulphate oxidation method (Valderrama, 1981). Chlorophyll-a was quantified by spectrophotometry using ethanol 95% as a solvent (Wintermans & De Mots, 1965; Jespersen & Christoffersen, 1987) after filtering a volume of 250-500 mL onto a glass fiber filters. Total suspended solids (TSS), volatile suspended solids (VSS - Organic) and fixed suspended solids (FSS - Inorganic) were determined through gravimetric analyses on samples filtered onto glass fiber filters (Rice et al., 2012). We used the criteria by Thornton & Rast (1993) to access the trophic states of the reservoirs According to them, semi-arid reservoirs with total phosphorus (TP) concentrations in water lower than 50 μg.L-1 and total chlorophyll-a lower than 15 μg.L-1 may be classified as mesotrophic. On the other hand those reservoirs with TP reaching values higher than 50 μg.L-1 and chlorophyll-a higher than 15 μg.L-1 may be considered as eutrophic.

The identification and quantification of phytoplankton community were performed with an optic and inverted microscope (400x magnification), whenever possible until species level through analyses of morphologic and morphometric characteristics. The individuals (cells, col-onies, and filaments) were enumerated in aleatory fields (Uhelinger, 1964), using the sedimenta-tion technique (Utermöhl, 1958). Then, at least 100 individuals of the most frequent species (P<0,05; Lund et al., 1958) were counted. The sedimented volumes were determined based on the concentration of algae and/or detritus. The biovolume (mm3L-1) was obtained based on ap-proximated geometric forms (Hillebrand, 1999; Fonseca et al., 2014) after counting 40-60 indi-viduals. Phytoplankton biomass (mg.L-1) was calculated by assuming that the unit of fresh

19 weight is equivalent to a mass of 1 mm3.l-1 = 1 mg.l-1 (Wetzel and Likens 2000). Species repre-senting more than 5% of the total biomass were included in the statistical analyses.

We considered a specie or genus dominant when their relative biomass represents more than 50% of the total biomass. To access environmental preferences of Cylindrospermopsis

raciborskii and Microcystis aeruginosa, we created the “Cylindrospermopsis abundance index”:

𝐶𝑦𝑙𝑖𝑛𝑑𝑟𝑜𝑠𝑝𝑒𝑟𝑚𝑜𝑝𝑠𝑖𝑠 𝑎𝑏𝑢𝑛𝑑𝑎𝑛𝑐𝑒 𝑖𝑛𝑑𝑒𝑥 = 𝐶. 𝑟𝑎𝑐𝑖𝑏𝑜𝑟𝑠𝑘𝑖𝑖 𝑏𝑖𝑜𝑚𝑎𝑠𝑠

𝐶. 𝑟𝑎𝑐𝑖𝑏𝑜𝑟𝑠𝑘𝑖𝑖 𝑏𝑖𝑜𝑚𝑎𝑠𝑠 + 𝑀. 𝑎𝑒𝑟𝑢𝑔𝑖𝑛𝑜𝑠𝑎 𝑏𝑖𝑜𝑚𝑎𝑠𝑠

The index was applied to all abiotic variables measured, in order to identify the envi-ronmental conditions where M. aeruginosa and C. raciborskii each specie dominate. The results of the index were displayed as represented bellow, where 0.5 means equal biomass for both species, 1.0 represents total dominance of Cylindrospermopsis raciborskii and 0.0 indicates total dominance of Microcystis aeruginosa.

S

A

A detrended correspondence analysis (DCA) was performed using the biological data in order to indicate the most suitable constrained ordination method for our data (Ter Braak & Prentice, 1988). In order to investigate the relationship between cyanobacteria assemblage and the envi-ronmental variability, we performed a Redundancy Analysis (RDA) using the relative biomass of each genus as the response variable and measured environmental variables as the explanatory variables. The abiotic variables used were selected based on their variance inflation factor (VIF); only variables with VIF bellow 10 were used to avoid multi-collinearity (Hair et al., 1995). The environmental variables comprised in the RDA were euphotic zone, depth, water surface temperature, total and dissolved phosphorus and nitrogen, volatile and fixed suspended solids. Abiotic data were transformed to log10(X+1), while the biotic data were converted by

100% 𝐶. 𝑟𝑎𝑐𝑖𝑏𝑜𝑟𝑠𝑘𝑖𝑖

100% 𝑀. 𝑎𝑒𝑟𝑢𝑔𝑖𝑛𝑜𝑠𝑎

20 Hellinger transformation (Legendre & Gallagher, 2001). Monte Carlo test using 999 permuta-tions was performed to test the significance of RDA axis. Linear Regressions were performed to access the effect of depth in the concentration of nutrients and dominance of cyanobacteria. The dependent variables were transformed to log10(X+1) and then checked to confirm that fits

normality criteria. The criteria of p < 0.05 was considered as statistically significant. All statisti-cal analyses were performed using R software (R Core Team, 2019).

R

3.1

L

All reservoirs had a high temporal variability in chemical, physical and biological parameters (Table 3). Also, reservoirs displayed a spatial gradient of eutrophication, where Sta. Cruz was the least and Pajuçara was the most enriched reservoir. Based on eutrophication threshold values (Thornton & Rast, 1993), Sta. Cruz and Prata reservoirs were classified as mesotrophic. Tabat-inga had total phosphorous concentration bellow eutrophication threshold and chlorophyll-a above it; thus, Tabatinga may be considered as a reservoir in transition to eutrophication. Whilst Encanto, Boq. Parelhas and Pajuçara were eutrophic (Table 3). As expected, Boq. Parelhas and Pajuçara had the highest phytoplankton biomass (643.13 and 469.78 mg.L-1 , Table 3) and Sta. Cruz had the lowest biomass (2.06 mg.L-1, Table 3). We assumed that all reservoirs were mixed, considering that surface and bottom water temperatures were equal (data not shown).

3.2

P

C

D

Cyanobacteria dominance increased following the trophic state. The mesotrophic reservoirs had phytoplankton dominance alternated among different algal and cyanobacteria groups during the study. Sta. Cruz was dominated by cyanobacteria only in January, while Prata reservoir had cyanobacteria dominance from October to December (Fig. 2A and 2B). Tabatinga had a more expressive dominance of cyanobacteria (Fig. 2C), while the eutrophic reservoirs (Encanto, Boq. Parelhas e Pajuçara) faced persistent dominance of cyanobacteria virtually during the entire year (Fig. 2D, Fig. 2E, Fig. 2F). Nevertheless the dominant species differed among reservoirs and along the sampling months. In Tabatinga and Boq. Parelhas reservoirs the dominant species were Planktolyngbya limnetica and Microcystis aeruginosa, respectively, while Encanto and Pajuçara reservoirs presented switches of dominance between Cylindrospermopsis raciboskii and Microcystis aeruginosa (Table 4).

21 Pa ju ça ra ₀.6 (0 .1 -1 ) 4 .5 2 (1 .3 -7) 28 (2 5 -30) 1 0 7 .5 6 (4 2 .9 2 -2 3 1 .8 9 ) 4 0 .7 1 (1 7 .8 5 6 7 .4 4 ) 3 4 6 2 .9 2 (1 3 2 8 -6 8 6 4 ) 2 7 3 6 .2 7 (1 3 7 0 - 5 5 7 4 ) 3 3 .3 4 (1 8 .3 6 -4 0 .2 9 ) 1 6 .0 2 (8 – 3 7 ) 5 .1 5 (2 .2 0 -1 0 .8 0 ) 1 0 .8 7 (3 .8 3 -2 8 .5 0 ) 7 0 .2 8 (1 0 .3 9 -193 -6 5 ) 9 7 .2 9 (7 .8 3 -4 6 9 .7 8 ) B o q u ei rã o Pa re lh a s 0 .5 (0 .1 -0 .6 ) 5 (3 .4 -6 .8 ) 26 (2 3 -28) 9 7 .5 (5 0 .2 4 -1 7 0 .7 6 ) 3 8 .8 (1 6 .7 8 -1 1 9 .7 5 ) 1 8 2 4 .8 3 (1 2 7 8 -2 3 8 3 ) 1 5 3 0 ( 1 2 1 2 -2 0 7 0 ) 2 0 .9 7 (1 0 .1 1 -3 8 .8 1 ) 1 5 .8 7 (8 .2 0 -2 7 .7 5 ) 9 .6 8 (3 .4 0 -1 9 .7 5 ) 6 .1 9 (4 .5 0 -8 .0 0 ) 2 9 .1 1 (4 .8 -5 1 .1 6 ) 1 3 6 .8 0 ( 4 .8 8 -6 4 3 .1 3 ) E n ca n to 0 .7 (0 .3 -1 .3 ) 5 .9 (3 .8 -8 .3 ) 28 (2 5 -31) 6 7 .5 7 ( 1 6 .7 4 -1 1 1 .8 6 ) 2 9 .5 1 ( 1 2 .8 3 -5 2 .3 2 ) 1 5 0 2 .2 6 (7 0 7 .9 0 -2 5 0 2 ) 1 0 4 6 .9 5 ( 7 4 6 .1 -1 3 2 4 ) 2 5 .9 4 (1 5 .9 -6 6 .2 9 ) 1 0 .4 1 (3 -3 4 .6 7 ) 3 .2 4 (0 .6 0 -12) 7 .1 7 (2 .2 5 -2 2 .6 7 ) 3 2 .5 6 (8 .9 9 -1 1 2 .7 1 ) 4 8 .4 2 (1 .5 6 -1 7 5 .8 1 ) T a b a ti n g a 0 .8 (0 .6 -0 .9 ) 5 (3 -7 ) 28 (2 5 -30) 4 5 .4 3 ( 3 3 .0 9 -5 6 .1 0 ) 2 3 .2 5 (1 2 .8 3 -4 5 .0 2 ) 2 1 5 0 .1 7 (1 6 4 0 -2 3 8 9 ) 1 8 0 9 .4 5 (1 4 7 3 -2 0 4 2 ) 4 8 .3 5 (3 7 .4 8 -6 6 .0 7 ) 1 6 .8 6 (1 0 .4 0 -3 2 ) 5 .4 6 (1 .6 0 -1 3 .5 0 ) 1 1 .4 0 (8 .5 5 -1 8 .5 0 ) 3 4 .9 9 (2 0 .2 1 -5 3 .5 6 ) 1 3 .3 4 ( 4 .7 0 -6 4 .4 8 ) Pr a ta 1 .4 (1 .2 -1 .6 ) 3 .9 (2 .1 -11 ) 28 (2 6 -31) 3 8 .7 4 (2 2 .8 6 -6 9 .2 5 ) 1 7 .0 3 (3 .5 5 -6 0 .8 3 ) 6 5 2 .6 5 (5 0 0 .3 -8 3 0 .3 ) 5 7 3 .7 3 (4 5 6 .1 7 4 2 .1 ) 1 8 .7 0 (7 .7 8 -2 9 .7 3 ) 6 .8 4 (2 .8 0 -1 2 .1 3 ) 3 .1 1 (0 .2 0 -8 .4 0 ) 3 .7 3 (2 -6 .8 1 ) 7 .5 8 (3 .7 7 -1 1 .6 5 ) 3 .9 0 (1 .0 6 -1 2 .6 9 ) S a n ta C ru z d o A p o d i 2 .8 (1 .3 -4 .5 ) 1 6 .6 (1 0 .9 -1 9 .3 ) 28 (2 6 -30) 2 2 .7 1 (1 2 .8 3 -3 5 .8 3 ) 1 0 .7 1 (1 .6 6 -2 0 .1 9 ) 6 9 0 .5 8 ( 5 4 3 .7 -8 0 2 .4 ) 6 0 3 .9 (5 1 0 .3 -7 4 8 .2 ) 3 2 .1 5 (1 8 .9 8 -4 3 .8 4 ) 3 .1 8 (1 .5 0 -6 .2 9 ) 1 .6 1 (1 -2 .5 7 ) 1 .5 7 (0 -3 .7 1 ) 2 .9 9 (0 .9 -1 0 .7 9 ) 0 .6 (0 .0 3 -2 .0 6 ) R eser v o ir s S ec ch i (m) _Zma x (m) _Temp _(°C) P to ta l ( µ g .L -1) P d isso lv ed ( µ g .L -1) N t o ta l ( µ g .L -1) N d isso lv ed ( µ g .L -1 ) T N: TP T S S ( m g .L -1) FSS ( m g .L -1) V S S ( m g .L -1) C h lo ro p h y ll -a (µ g .L -1 ) B io m a ss (mg .L -1) Ta b le 3 De sc rip ti v e sta ti stics (a v era g e, m ax imu m a n d m in imu m v alu es, b etwe en p are n th ese s) o f li m n o lo g ica l v ariab les fro m e ac h re se rv o ir d u ri n g th e y ea r o f 2 0 1 7 . Th e tem p era tu re re fe rs to su rfa ce m ea su re .

22

Fig. 2 Phytoplankton taxonomic composition shown as relative biomass of each phytoplankton class during the year of 2017 for each studied reservoir.

0 20 40 60 80 100 1 2 3 4 5 6 7 8 9 10 11 12 R e lat iv e B iom ass Months

Sta. Cruz

Cyanobacteria Chlorophycea Cryptophyceae Bacillariophycea Euglenophyceae Dynophyceae

(A) 0 20 40 60 80 100 1 2 3 4 5 6 7 8 9 10 11 12 R e lat iv e B iom ass Months

Prata

Tabatinga

Cyanobacteria Chlorophycea Cryptophyceae Bacillariophycea Synurophyceae (C) 0 20 40 60 80 100 1 2 3 4 5 6 7 8 9 10 11 12 R e lat iv e B iom ass Months

Encanto

Cyanobacteria Chlorophycea Cryptophyceae Bacillariophycea Dynophyceae Euglenophyceae

(D) 0 20 40 60 80 100 1 2 3 4 5 6 7 8 9 10 11 12 R e lat iv e B iom ass Months

Boq. Parelhas

Cyanobacteria Chlorophycea Cryptophyceae Dynophyceae Bacillariophycea (E) 0 20 40 60 80 100 1 2 3 4 5 6 7 8 9 10 11 12 R e lat iv e B iom ass Months

Pajuçara

Cyanobacteria Chlorophycea Cryptophyceae Bacillariophycea Euglenophyceae

23

Table 4 Cyanobacteria dominant genera in each studied reservoir study (2017). Symbols meaning: NA refers to data not available; 0 means no cyanobacteria dominance; ● Cylindrospermopsis; ○ Microcystis; ■ Planktolyngbya, □ Aphanocapsa; ▲ Pseudanabaena; ⁑ Chrooccocus.

Sta. Cruz Prata Tabatinga Encanto Boq. Parelhas Pajuçara

January ⁑ 0

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3.3

R

EXPLAN-ATORY VARIABLES

The eight environmental variables, considered for the RDA analysis, explained 40% of the variation in the dominance of four cyanobacteria genera, Cylindrospermopis, Microcystis,

Planktolyngby, Aphanocapsa (Fig. 3, R2 = 0.404; R2 adjusted = 0.301). The axes 1 and 2 ex-plained 13,8% and 8,9%, respectively. The Monte Carlo test indicated that both axes were statistically significant (axis 1 p<0.001; axis 2 p<0.01). The RDA coefficients indicated that Zeu (0.74), TP (-0.67), TDP (-0.57) and TDN (-0.62) were mostly correlated to axis 1, while Zmax (-0.94), VSS (0.665) and FSS (0.464) were correlated to axis 2. In the negative side of axis 1, the dominance of Cylindrospermopis and Microcystis was related to higher concentrations of TP, TDP,TDN and reduced Zeu in samples from Boq. Parelhas, Encanto and Pajuçara reser-voirs. In the positive side of axis 2, Planktolyngbya was related to higher concentrations of VSS, FSS and lower Zmax considering Tabatinga and Prata reservoirs. Regarding the negative side, Aphanocapsa was related to environments variables with higher Zmax and low concentra-tion of suspended solids (VSS and FSS). The RDA indicated depth as the variable that most explained the variation in cyanobacteria dominance. The linear regressions indicated that total phosphorus and phytoplankton biomass are significantly and negatively correlated with depth (Fig. 4A and 4B).

24

Fig. 3 Redundancy Analysis (RDA) applied to the relative biomass of cyanobacteria and abiotic variables; (Zeu) Eupho-tic zone; (Zmax) Maximum depth; (Temp) Water surface temperature; (TP) Total phosphorous; (TDP) Total dissolved phosphorous; (TN) Total nitrogen; (TDN) Total dissolved nitrogen; (VSS) volatile suspended solids and (FSS) fixed suspended solids in the six reservoirs studied.

Fig. 4 Linear regression between maximum depth (Zmax) and (A) total phosphorous (µg.L-1_{), (B) Phytoplankton}

biomass (mg.L-1). Y axis is shown in logarithmic scale. Data represent all samples of the six study reservoirs from January to December of 2017. The red lines show the fit for the regression model.

(A) _(B)

Y= 4.061x-0.268 Adj R2_{= 0.324***}

Y=4.410x-0.074 Adj R2= 0.269***

25

3.4

C

M

Cylindrospermopsis and Microcystis were the most representative genera of cyanobacteria

dur-ing the study bedur-ing mainly represented by Cylindrospermopsis raciborskii and Microcystis

ae-ruginosa. The “Cylindrospermopsis abundance index” was applied to differentiate the

environ-mental condition where each one dominated, due to a distinct pattern of dominance switch be-tween C. raciborskii to M. aeruginosa in Encanto and Pajuçara reservoirs. We found that M.

aeruginosa only dominated in concentrations of TP under 125 µg.L-1 (Fig. 5A), TSS 20 mg.L-1 (Fig. 5B), and TND 2500 µg.L-1 (Fig. 5C); also, M. aeruginosa did not dominate in environ-ments with euphotic zone bellow 3 meters (Fig. 5D), while C. raciborskii dominated in envi-ronments with conditions bellow and above these thresholds. In general, co-occurrence be-tween both bloom forming cyanobacteria was rare.

(A) (B)

(C) (D)

Fig. 5 Cylindrospermpsis Abundance Index in relation to (A) dissolved nitrogen (µg.L-1), (B) Total phosphorous (µg.L-1), (C) Total suspended solids (mg.L-1), (D) Euphotic zone (m). Black dash lines on horizontal represent equal

biomass for both species, 0 means 100% dominance of Microcystis and 1 means 100% dominance of

26

D

As the trophic state changes towards eutrophic conditions, cyanobacteria blooms moves from intermittent (mesotrophic reservoirs) to persistent (eutrophic reservoirs) and the relative contri-bution of other algal groups decline resulting in loss of phytoplankton diversity. Although cya-nobacteria blooms depends on bottom-up and top-down forces, the availability of nutrients, N and P, is often the key driver regulating their magnitude (Paerl, 2017). All eutrophic reservoirs presented constant excessive growth of cyanobacteria, in agreement with the view that persis-tent blooms are likely to install in environments that are suitable for cyanobacteria optimal growth, where nutrients, temperature and light were constantly favourable, making tropical res-ervoirs even more sensible to eutrophication (Figueredo & Giani, 2009; Soares et al., 2009; Figueredo et al., 2016; Lind et al., 2016; Batista et al., 2018).

Besides eutrophication has promoted cyanobacterial dominance through the whole stud-ied year, the combination of environmental conditions shaped the structure of the cyanobacteria assemblage by selecting the best adapted, this explaining temporal and spatial differences be-tween dominant species found in our reservoirs. Planktolyngbya has been associated to turbid

environments, with mixed layers and low values of phosphate in Brazilian semiarid reservoirs (Pinto & Becker, 2015; de Sousa Barroso et al., 2018). These patterns corroborates with our findings, since Planktolyngbyawas related to high concentrations of suspended solids (organic plus inorganic) and low maximum zone, dominating in practically all year in Tabatinga reser-voir, that presented the highest value of total suspended solids and the lowest P value among the eutrophic reservoirs, favouring the dominance of shade-adapted cyanobacteria. Meanwhile,

Aphanocapsa is typical from eutrophic environments (Padisák et al., 2009), however it was

found in mesotrophic reservoirs, although not as the dominant species.

Even though the RDA only indicated that C. raciborskii and M. aeruginosa thrives in eutrophic environments with reduced euphotic zone, the “Cylindrospermopsis abundance in-dex” we created was successful to differentiate the environmental conditions favouring each species, helping to understand the abiotic conditions driving the switch of dominance between them. C. raciborskii and M. aeruginosa are bloom-forming species with potential toxicity commonly found in eutrophic environments in Brazil, still they have different environmental preferences that affect their occurrence resulting in rare co-dominance cases (Soares et al., 2013). Our results reinforce this view, because C. raciborskii and M. aeruginosa barely appears in co-dominance and the “Cylindrospermopsis abundance index” showed a slightly difference in environmental condition where each one dominated.

27

C. raciborskii occurred in higher concentrations of total phosphorus compared to M. aeruginosa. Soares et al. (2013) also found that C. raciborskii dominance was associated

with higher concentrations of total phosphorus than M. aeruginosa. Consistent with this, Chislock et al. (2014) showed that C. raciborskii dominate under a wide range of phosphorus levels. In addition, C. raciborskii possess critical requirement of phosphorus lower than

M. aeruginosa. On the other hand, M. aeruginosa have demonstrated a higher maximum

up-take rate than C. raciborskii (Marinho et al., 2013), explaining dominance of both at low TP. Smith, (1983) stipulated that nitrogen-fixing species would dominate in habitats deficient in inorganic nitrogen. Nevertheless, C. raciborskii dominated in different concentration of dis-solved nitrogen, including N-sufficient conditions. The capacity to fix nitrogen has been in-voked to explain C. raciborskii successful outcome; however, some studies have pointed out that N-fixation was unlikely to be the mechanism providing competitive advantage in eutrophic tropical systems (Huszar et al., 2000; Burford et al., 2006; Soares et al., 2013). Laboratory stud-ies showed that C. raciborskii preferentially uses ammonium and nitrate over atmospheric ni-trogen (Sprőber et al., 2003) because it is energetically advantageous.

C. raciborskii has a high level of flexibility with respect to light and nutrients

promot-ing growth. The combination of a flexible strategy with respect to environmental conditions, and variability in strain response explain its success globally (Burford et al., 2016). In condi-tions of low total suspended solids C. raciborskii and M. aeruginosa dominated. However the increasing of total suspended solids, and as consequence the reduction of euphotic zone, fa-voured C. raciborskii, that has been long recognized as a shade-tolerant specie (Padisák & Reynolds, 1998) typical from turbid and mixed environments (Reynolds, 2002). In opposite, M.

aeruginosa did not appear in environment with high total suspended solids. This species is

sen-sitive and may grows slower under low total light (Reynolds, 2002). Batista et al. (2018) also found out that higher water transparency was more associated to Microcystis than to

Cylin-drospermopsis explaining the switch pattern between them.

Warmer temperatures favour surface bloom-forming cyanobacterial genera because they are well-adapted to hot conditions(Paerl & Huisman, 2008). In addition warming intensify ver-tical stratification, which would also favour buoyant cyanobacteria’s taxa (Paerl, 2017). Cyano-bacterial dominance has been related to high temperatures even in the tropics (Bouvy et al., 2000; Huszar et al., 2000; Soares et al., 2009). However, we did not found this relation. Cyano-bacteria were capable of phytoplankton dominance at different temperatures (unshown data).It is important to highlight that all reservoirs are located in the same region and it presents slightly seasonal temperature variation due to the semi-arid climate. In addition, some species, as C.

28 raciborskii grows at a wide range of temperatures (Bonilla et al., 2016). Therefore, further stud-ies are required to clarify the relationships between temperature and cyanobacteria dominance in semi-arid regions.

Arid and semi-arid regions are characterized by prolonged drought and short rainy peri-ods altering water levels and interfering in chemical and physical water characteristics (Castro Medeiros et al., 2015). A variety of studies have demonstrated that drought increase total phos-phorous, nitrogen and as result favour cyanobacteria dominance (Aldridge, 2014; Brasil et al., 2016; Rocha Junior et al., 2018; Havens et al., 2019). Our results are aligned with this pattern. The reduction of maximum depth had a profound effect on nutrient, phosphorous concentration, and influenced the phytoplankton community, resulting in cyanobacteria dominance. The in-verse relation of phosphorous and depth, could be allied with internal loading through anoxia considering that semi-arid reservoirs are susceptible to it (Cavalcante et al., 2018). However, further studies are required to clarify the relation between depth and internal loading.

Hence, drought leading to cyanobacterial dominance tends not be the rule in shallow tropical reservoirs (Costa et al., 2019; Crossetti et al., 2019). It consequences appear to depend strongly on environment characteristics, such as morphometry, sediment type, water retention time, quality of inlet water and climate (Bakker & Hilt, 2016). Climate change predictions for semiarid lands of Northeast Brazil suggests rainfall reductions, temperature increases and water deficits and longer dry spells, leading to even more several drought (Marengo & Bernasconi, 2015) and the consequences of the increase of dryness for phytoplankton community remains controversial for these areas.

In general, our work demonstrated that eutrophic semi-arid reservoirs are even more susceptible to present persistent dominance of cyanobacteria. Also, that depth has an important role that influences the availability of resources and as consequence phytoplankton community structure. Nitrogen and phosphorus were the main abiotic drivers of Cylindrospermopsis and

Microcystis dominance; however, light is likely to be the major factor regulating the switch

between them. In conclusion, our results indicate that eutrophic semi-arid reservoirs are even more sensible to eutrophication and urge demands effective efforts to control cyanobacteria dominance and improve water quality, including reduction of external nutrient loading (nitrogen and phosphorous) and introduce depth as an important parameter in water management per-spective.

29

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