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Data Paper (Biosciences)
Diatom diversity and distribution in Madeira Island streams (Portugal)
expand article infoCatarina Ritter, Pedro M. Raposeiro, Vítor Gonçalves
‡ CIBIO, Research Center in Biodiversity and Genetic Resources, InBIO Associate Laboratory / Faculty of Sciences and Technology, University of the Azores, Ponta Delgada, Portugal
Open Access

Abstract

Background

Here, we present the data obtained from the samples collected in a field campaign during the spring of 2015 which aims for a better understanding of the diversity and distribution patterns of freshwater diatoms in Madeira Island. Following European and Portuguese standards and recommendations for routine diatom sampling and analysis, we collected samples in 40 sites, distributed in 27 permanent streams and identified the diatom species present, using general diatom floras and studies in Portuguese freshwater diatoms.

New information

Little is known about the diversity and distribution of freshwater diatom assemblages from Madeira Archipelago. This study reports a survey in 40 sites in Madeira Island distributed in 27 permanent streams. A total of 965 diatom (Bacillariophyta) occurrences were recorded, belonging to 130 different taxa from 44 genera and 27 families. The families with the highest number of occurrences were Bacillariaceae (176), Achnanthidiaceae (135) and Naviculaceae (133). The two diatom endemisms, described previously in Madeira Island (Lange-Bertalot 1993), Nitzschia macaronesica Lange-Bertalot and Navicula madeirensis Lange-Bertalot, were only observed in a small number of sites, located mostly at Laurissilva forest. Sixty species are new records, not only to Madeira Island, but also to the Madeira Archipelago.

Keywords

Bacillariophyta, oceanic islands, freshwater systems, new records, diatom occurrences

Introduction

Diatoms (Bacillariophyta) are microscopic algae and one of the most abundant and diverse group of aquatic, pigmented single-celled photosynthetic eukaryotes, which can be found in almost every type of aquatic environment around the globe (Kociolek 2007, Pla et al. 2016). These microalgae are characterised by an outer silica wall (frustule) that makes them easy to collect and preserve for later identification. Benthic diatoms, in particular, are important contributors to primary production in streams and are widely used as indicators for monitoring the ecological status of aquatic systems and also for past environmental and climatic reconstructions (Battarbee et al. 2006), since many species have distinct ecological optima and narrow tolerance (Cohen 2003). Diatom communities inhabiting streams have been studied in several regions of the globe (e.g. Tison et al. 2005, Grenier et al. 2006, Mora et al. 2017, Falasco et al. 2016, Liu et al. 2019), including insular streams (Delgado et al. 2012, Delgado et al. 2013, Kopalová et al. 2011, Kopalová and van de Vijver 2013, Gonçalves et al. 2015, Vasselon et al. 2017).

Despite their great importance, current knowledge about the freshwater diatoms on insular streams of Madeira Archipelago is limited in comparison with freshwater macroinvertebrates (Hughes et al. 1998, Hughes and Furse 2001, Hughes 2006), bryophytes (Sérgio and Fontinha 1994, Sérgio et al. 2006, Sim-Sim et al. 2008, Sim-Sim et al. 2014, Sim-Sim et al. 2011, Boch et al. 2019) and hyphomycetes communities (Raposeiro et al. 2020). Although diatoms from the Madeira Archipelago have been a matter of study for more than 150 years (Grunow 1867, De-Toni 1891, De-Toni 1892, Zimmermann 1909, Zimmermann 1911, Schodduyn 1927, Mölder 1947, Lange-Bertalot 1993, Kaufmann et al. 2015, Gonçalves et al. 2016), including the description of two regional endemisms, little is known about the regional overall diversity of these microalgae in Madeira Island. The importance of insular freshwater studies of microalgal diversity is centred around the concept that these ecosystems tend to be less complex, providing much potential for testing ideas about biogeographic theory and species distribution limits (Flower 2005).

Here, we provide a detailed dataset that contains freshwater diatom occurrences collected during a field campaign on Madeira Island, increasing the knowledge on the epilithic diatom inhabiting permanent streams in Madeira Island. Our purpose is to release this valuable dataset, since no similar datasets have been previously published for Madeira Archipelago and it constitutes a relevant tool of comparison for aquatic ecologists, for example, biogeographic patterns, climate change or other studies on oceanic islands.

Project description

Title: 

Diatom diversity and distribution in Madeira Island streams (Portugal)

Personnel: 

Collections were undertaken and occurrence data recorded during the spring of 2015 in Madeira Island. The collectors were Pedro Raposeiro and Vitor Gonçalves. Identifications were made by Catarina Ritter and supervised by Vitor Gonçalves.

Study area description: 

The Madeira Archipelago is an oceanic archipelago located in the North Atlantic between latitudes 32°24' and 33°07'N and longitudes 16°16' and 17°16'W (Fig. 1). Madeira Island is the highest (Pico Ruivo - 1861 m) and largest island (~ 740 km2) of the archipelago and about 90% of its area is higher than 500 m above sea level (Ribeiro 1985). Madeira Island presents a high diversity of habitat types, including the largest surviving area of Laurissilva forest in Macaronesia, classified as a UNESCO World Natural Heritage site (IUCN 1999). Due to its oceanic condition, Madeira Island presents a mild temperate oceanic climate strongly influenced by winds from the NE and the Canary Islands current, presenting a relative humidity between 55-75% and annual rainfall between 500 and 1,000 mm (AEMET & IM 2012). An important aspect of the climate in Madeira Island is the persistent nebulous covering of fog, which normally exists in high altitude resulting in an important source of groundwater recharge (Prada et al. 2005). Under this mild temperate oceanic climate, groundwater hydrology is essential for surface water and for the persistence and functioning of the insular aquatic ecosystems as a high number of the permanent streams are fed by springs.

Figure 1.  

Geographical location of the study stream sites. a. Madeira Archipelago in the Atlantic Ocean highlighted by a red square; b. Madeira Island in the Madeira Archipelago; c. Studied stream sites.

Madeira Island comprises approximately 126 catchments and 200 streams presenting a typical radial drainage pattern common in oceanic islands (Marques 1994). According to Prada et al. 2005, the hydrographic network present in the Island is characterised by deep narrow valleys with a typical U-transverse profile as these are still in a young phase. Most of the streams have a torrential character with high flow rates (Hughes 2006).

Sampling methods

Study extent: 

Epilithic freshwater diatoms (Bacillariophyta Karsten 1928) from 40 sites (MAD01 – MAD40) from 27 permanent streams in Madeira Island.

Sampling description: 

In the spring of 2015, epilithic biofilm samples were collected in 40 sites (MAD01 – MAD40) from 27 permanent streams in Madeira Island (Table 1). The sampling sites ranged in altitudes (low, medium and high) and land-uses (natural, agricultural and urban) (Figs 2, 3, 4). For diatom analysis, samples were prepared following the European (Kelly et al. 1998, European Committee for Standardization 2003, European Committee for Standardization 2004) and national recommendations (INAG 2008). Epilithic diatoms were taken from stones with a toothbrush in each sampling site (Fig. 5). Immediately after collection, diatom samples were fixed with formalin at 4% final concentration. Permanent slides were prepared with Naphrax® and at least 400 valves per sample were counted and identified at the lowest taxonomic level possible under oil-immersion phase contrast light microscopy using a Leica DM2500 (Leica Microsystems GmbH, Welzlar, Germany).

Table 1.

Sampling codes and location of the forty studied stream sites on Madeira Island.

Sampling code Stream Municipality Sampling date Latitude(ºN) / Longitude(ºW) Altitude(m)
MAD01 Ribeira dos Socorridos Câmara de Lobos 28/04/2015 32.66319, -16.9606 85
MAD02 Ribeira Brava Ribeira brava 28/04/2015 32.73395, -17.021 409
MAD03 Ribeira da Vargem São Vicente 28/04/2015 32.76807, -17.0305 450
MAD04 Ribeira de São Vicente São Vicente 28/04/2015 32.77415, -17.0245 325
MAD05 Ribeira Grande São Vicente 28/04/2015 32.77599, -17.0244 311
MAD06 Ribeira Grande São Vicente 28/04/2015 32.28433, -16.7232 60
MAD07 Ribeira Brava São Vicente 28/04/2015 32.75216, -17.0244 903
MAD08 Ribeira Brava São Vicente 28/04/2015 32.74842, -17.0257 833
MAD09 Ribeira dos Socorridos Câmara de Lobos 29/04/2015 32.74522, -16.9591 826
MAD10 Ribeira da Gomeira Câmara de Lobos 29/04/2015 32.74572, -16.9646 725
MAD11 Corgo da Ribeira de Anéis Câmara de Lobos 29/04/2015 32.74059, -16.9652 780
MAD12 Ribeira do Cidrão Câmara de Lobos 29/04/2015 32.72749, -16.9653 597
MAD13 Ribeira do Machico Machico 29/04/2015 32.71876, -16.7642 10
MAD14 Ribeira do Juncal Machico 29/04/2015 32.77081, -16.8289 36
MAD15 Ribeira do Juncal Machico 29/04/2015 32.76142, -16.8376 187
MAD16 Ribeira do Faial Machico 29/04/2015 32.74741, -16.8313 560
MAD17 Ribeira do Machico Machico 29/04/2015 32.73962, -16.8347 624
MAD18 Ribeira Primeira Machico 29/04/2015 32.73101, -16.8388 791
MAD19 Ribeira do Machico Machico 29/04/2015 32.73715, -16.8493 877
MAD20 Ribeira de Santa Cruz Santa Cruz 29/04/2015 32.68695, -16.792 7
MAD21 Ribeira da Janela Porto Moniz 30/04/2015 32.85522, -17.1537 81
MAD22 Ribeira do Alecrim Porto Moniz 30/04/2015 32.75164, -17.1121 1391
MAD23 Ribeira da Janela Porto Moniz 30/04/2015 32.7603, -17.1241 1089
MAD24 Ribeira da Janela Porto Moniz 30/04/2015 32.76077, -17.1283 1041
MAD25 Ribeira dos Cedros Porto Moniz 30/04/2015 32.76582, -17.1256 899
MAD26 Ribeira da Janela Porto Moniz 30/04/2015 32.76503, -17.1324 1003
MAD27 Ribeira da Janela Porto Moniz 30/04/2015 32.76191, -17.1252 1271
MAD28 Ribeira do Alecrim Porto Moniz 30/04/2015 32.7535, -17.129 1182
MAD29 Ribeira Frio Santana 01/05/2015 32.72254, -16.8897 846
MAD30 Córrego do Arrochete Santana 01/05/2015 32.73768, -16.8864 637
MAD31 Ribeira da Metade Santana 01/05/2015 32.74293, -16.9064 686
MAD32 Ribeira das Lajes Santana 01/05/2015 32.73838, -16.9057 23
MAD33 Ribeira de São Roque do Faial Santana 01/05/2015 32.78725, -16.8497 42
MAD34 Ribeira Seca Santana 01/05/2015 32.78758, -16.8505 103
MAD35 Ribeira de São Jorge Santana 01/05/2015 32.81442, -16.9044 121
MAD36 Ribeira dos Arcos Santana 01/05/2015 32.81342, -16.904 517
MAD37 Ribeira de São Jorge Santana 01/05/2015 32.82849, -16.8978 21
MAD38 Ribeira de Santa Luzia Funchal 02/05/2015 32.67818, -16.9182 25
MAD39 Ribeira da Fonte do Bugio Calheta 02/05/2015 32.72153, -17.1784 22
MAD40 Ribeira da Ponta do Sol Ponta do Sol 02/05/2015 32.6803, -17.1052 85
Figure 2.  

Sampling site representing the natural land-use. Located at Ribeira Primeira, Santo António da Serra, Machico (MAD18).

Figure 3.  

Sampling site representing the agricultural land-use. Located at Ribeira de São Jorge, Santana (MAD37).

Figure 4.  

Sampling site representing an urban land-use. Located at Ribeira da Janela, Porto Moniz (MAD21).

Figure 5.  

Collecting epilithic diatoms from stones with a toothbrush at each sampling site.

Quality control: 

Diatom morphometric features were determined by photomicrography (Leica DFC495) with the aid of image analysis software (LAS version 3.8.0). Diatom identification was based on reference diatom floras (e.g. Krammer and Lange-Bertalot 1986, Krammer and Lange-Bertalot 1988, Krammer and Lange-Bertalot 1991, Krammer and Lange-Bertalot 2000, Krammer and Lange-Bertalot 2002, Lange-Bertalot et al. 2017), as well as on recent bibliographic sources, including the series “Diatoms of Europe”, “Bibliotheca Diatomologica”, relevant taxonomic papers (e.g. Trobajo et al. 2013, Wetzel et al. 2015) and studies in Portuguese freshwater diatoms (Novais et al. 2014). Nomenclatural and taxonomic status used here follows Algaebase (Guiry and Guiry 2020).

Step description: 

The data have been published as a Darwin Core Archive (DwC-A), which is a standardised format for sharing biodiversity data as a set of one or more data tables. The core data table contains 965 occurrences with 130 records (Ritter et al. 2020).

Geographic coverage

Description: 

Madeira Island, Madeira Archipelago, Macaronesia, Portugal.

Coordinates: 

32.6228N and 32.8815N Latitude; -17.2739W and -16.6487W Longitude.

Taxonomic coverage

Description: 

All diatoms were identified to genus or species level. In total, 130 taxa were identified belonging to five subclasses, 17 orders, 27 families and 44 genera distributed in the subphylums Coscinodiscophytina and Bacillariophytina.

Taxa included:
Rank Scientific Name Common Name
phylum Bacillariophyta Diatom

Usage licence

Usage licence: 
Open Data Commons Attribution License
IP rights notes: 
This work is licensed under a Creative Commons Attribution (CC-BY) 4.0 License.

Data resources

Data package title: 
Diatom distribution in Madeira Island streams (Portugal)
Number of data sets: 
1
Data set name: 
Diatom distribution in Madeira Island streams (Portugal)
Data format: 
Darwin Core Archive
Data format version: 
1.2
Description: 

This paper presents data from freshwater diatoms surveys developed in Madeira Island in 2015. The dataset has been published as a Darwin Core Archive (DwC-A), which is a standardised format for sharing biodiversity data as a set of one or more data tables. The core data table contains 40 events (eventID), 965 occurrences (occurrenceID) with 130 taxa (taxonID). The number of records in the data table is illustrated in the IPT link. This IPT archives the data and thus serves as the data repository. The data and resource metadata are available for downloading in the downloads section.

Column label Column description
scientificNameAuthorship The authorship information for the scientificName.
type The nature of the resource.
basisofRecord The specific nature of the data record.
occurrenceID Identifier of the record, coded as a global unique identifier.
eventID Identifier of the event, unique for the dataset.
eventDate Time interval when the event occurred.
continent Continent of the sampling site.
waterBody Water body of the sampling site.
islandGroup Island group of the sampling site.
island Island from the Island Group of the sampling site.
country Country of the sampling site.
countryCode Code of the country where the event occurred.
municipality Name of the municipality where the event occurred.
locality Name of the locality where the event occurred.
decimalLatitude The geographic latitude of the sampling site.
decimalLongitude The geographic longitude of the sampling site.
geodeticDatum The spatial reference system upon which the geographic coordinates are based.
taxonID The identifier for the set of taxon information (data associated with the Taxon class). Specific identifier to the dataset.
scientificName The name with authorship applied on the first identification of the specimen.
acceptedNameUsage The specimen accepted name, with authorship.
kingdom Kingdom name.
phylum Phylum name.
class Class name.
order Order name.
family Family name.
genus Genus name.
specificEpithet The name of the first or species epithet of the scientificName.
infraspecificEpithet The name of the lowest or terminal infraspecific epithet of the scientificName, excluding any rank designation.
taxonRank The taxonomic rank of the most specific name in the scientificName.
coordinateUncertaintyInMetres The indicator for the accuracy of the coordinate location in metres, described as the radius of a circle around the stated point location.

Additional information

Analysis

This study presents 965 diatom (Bacillariophyta) occurrences in 40 sites in Madeira Island, belonging to 130 different taxa from 44 genera, 27 families, 4 suborders, 17 orders, 5 subclasses, 3 classes and 2 subphyllums (Table 2).

Table 2.

Main taxonomic figures.

Orders Families Genera Total taxa Total species New records Madeira endemisms
Aulacoseirales 1 1 2 1 1
Bacillariales 1 2 22 21 10 1
Cocconeidales 2 6 18 16 9
Cymbellales 3 7 18 18 8
Eunotiales 1 1 4 3 3
Eupodiscales 1 1 1 1
Fragilariales 2 3 14 14 9
Licmophorales 1 1 1 1
Mastogloiales 1 2 3 2 3
Melosirales 2 2 2 2
Naviculales 7 12 2 31 13 1
Rhopalodiales 1 1 31 2
Stephanodiscales 1 1 2 2 2
Surirellales 1 1 1 1 1
Tabellariales 1 2 3 3
Thalassiophysales 1 1 4 3 1

The subphylum Coscinodiscophytina, represented by one class, two orders and three families, accounted for 1.5% of the total occurrences, while the subphylum Bacillariophytina registered 98.4% of the total occurrences. With two classes and four subclasses, most occurrences (815) were registered in the Bacillariophycidae subclass.

The families with the highest number of occurrences were Bacillariaceae (176), Achnanthidiaceae (135) and Naviculaceae (133). However, the families with the highest number of taxa were Bacillariaceae (23), Naviculaceae (15), Achnanthidiaceae (15) and Rhoicospheniaceae (10). The families with lower occurrences (< 5) were Surirellaceae (1), Diploneidaceae (2), Aulacoseiraceae (2), Eupodiscaceae (2) and Stephanodiscaceae (2). However, the families with the smallest number of diatom taxa were Eupodiscaceae (1), Naviculales incertaesedis (1), Surirellaceae (1) and Ulnariaceae (1).

The genera with the highest number of occurrences were Nitzschia (174), Navicula (130), Planothidium (73), Cocconeis (54), Sellaphora (51), Fragilaria (51) and Gomphonema (50). The other 37 genera had less than 50 occurrences. The genera with the highest number of taxa were Nitzschia (22) and Navicula (13).

Achnanthidium minutissimum (Kützing) Czarnecki and Planothidium lanceolatum (Brébisson ex Kützing) Lange-Bertalot were the most frequent species occurring in 38 from 40 sites (MAD20 and MAD13 were the exceptions). Nitzschia soratensis E.A.Morales & M.L.Vis (37 sites), Cocconeis placentula var. euglypta (Ehrenberg) Grunow (36 sites), Amphora pediculus (Kützing) Grunow ex A.Schmidt (34 sites), Navicula reichardtiana Lange-Bertalot (33 sites) and Ulnaria ulna (Nitzsch) Compère (31 sites) were amongst the most ubiquitous diatoms.

A total of 45 diatom taxa occurring at only one sampling site were considered rare. These include benthic species, such as Achnanthes minuscula Hustedt, Achnanthidium pyrenaicum (Hustedt) H.Kobayasi, Adlafia minuscula (Grunow) Lange-Bertalot, Adlafia multnomahii E.A.Morales & M.Lee, Caloneis silicula (Ehrenberg) Cleve, Encyonema amanianum Krammer, Eunotia arcus Ehrenberg, Eunotia paludosa Grunow, Frustulia vulgaris (Thwaites) De Toni, Gomphonema augur Ehrenberg, Luticola mutica (Kützing) D.G.Mann, Nitzschia brevissima Grunow, Nitzschia filiformis (W.Smith) Van Heurck, Planothidium daui (Foged) Lange-Bertalot, Rhopalodia gibba (Ehrenberg) Otto Müller, amongst others; planktonic species, such as Aulacoseira granulata (Ehrenberg) Simonsen; tychoplanktonic species, such as Fragilaria capucina Desmazières, Pseudostaurosira elliptica (Schumann) Edlund, Morales & Spaulding, Pseudostaurosira robusta (Fusey) D.M.Williams & Round, Pseudostaurosira subconstricta (Grunow) Kulikovskiy & Genkal, Staurosirella lapponica (Grunow) D.M.Williams & Round, Stephanodiscus minutulus (Kützing) Cleve & Möller, Stephanodiscus parvus Stoermer & Håkansson; and aerophilic species, such as Diploneis elliptica (Kützing) Cleve, Diploneis praetermissa Lange-Bertalot & A.Fuhrmann, Humidophila brekkaensis (Petersen) R.L.Lowe, Kociolek, J.R.Johansen, Van de Vijver, Lange-Bertalot & Kopalová and Orthoseira roeseana (Rabenhorst) Pfitzer.

Another 32 diatom taxa were considered occasional, occurring in two to five sampling sites. These included benthic species, such as Achnanthidium gracillimum (F.Meister) Lange-Bertalot, Amphora inariensis Krammer, Craticula molestiformis (Hustedt) Mayama, Eunotia implicata Nörpel, Lange-Bertalot & Alles, Gomphonema acuminatum Ehrenberg, Gomphosphenia lingulatiformis (Lange-Bertalot & E.Reichardt) Lange-Bertalot, Grunowia solgensis (A.Cleve) Aboal, Hippodonta capitata (Ehrenberg) Lange-Bertalot, Metzeltin & Witkowski, Luticola goeppertiana (Bleisch) D.G.Mann ex J.Rarick, S.Wu, S.S.Lee & Edlund, Navicula madeirensis Lange-Bertalot, Navicula recens (Lange-Bertalot) Lange-Bertalot, Nitzschia recta Hantzsch ex Rabenhorst, Planothidium amphibium C.E.Wetzel, L.Ector & L.Pfister, Platessa oblongella (Østrup) C.E.Wetzel, Lange-Bertalot & Ector, Pleurosira laevis (Ehrenberg) Compère, Psammothidium hustedtii (Krasske) S.Mayama, planktonic species, such as Fragilaria gracilis Østrup and the tychoplanktonic species Staurosirella pinnata (Ehrenberg) D.M.Williams & Round and Tabellaria flocculosa (Roth) Kützing.

The mean number of taxa per sample was 24.1 ± 1.1 SE taxa. MAD40, MAD14, MAD26 and MAD23 were the samples with the highest number of taxa, 49, 38, 34 and 32, respectively. The samples with the lowest number of taxa were MAD20, MAD19 and MAD09 with 11, 14 and 15 taxa, respectively.

The two diatom endemisms, described previously in Madeira Island (Lange-Bertalot, 1993), Nitzschia macaronesica Lange-Bertalot and Navicula madeirensis Lange-Bertalot, were only observed in a small number of sites. Nitzschia macaronesica was present in 10 sites: Ribeira do Cidrão (MAD12), Ribeira do Juncal (MAD14), Corgo da Ribeira dos Aneis (MAD16), Ribeira da Janela (MAD23, MAD24 and MAD26), Ribeira dos Cedros (MAD25), Ribeira da Metade (MAD31), Ribeira das Lages (MAD32) and Ribeira de São Jorge (MAD37). This endemism appeared in sites with a richer diatom community (mean number of taxa of 28.9 ± 1.5 SE), located mostly at high altitude and was associated with Navicula cryptocephala, Navicula reichardtiana, Nitzschia soratensis and Ulnaria ulna, apart from the ubiquitous Achnanthidium minutissimum, Amphora pediculus, Cocconeis placentula var. euglypta and Planothidium lanceolatum.

Navicula madeirensis occurred only in four sites from different permanent streams: Ribeira do Faial (MAD16), Ribeira do Machico (MAD19), Ribeira do Alecrim (MAD28) and Ribeira da Fonte do Bugio (MAD39). These four sites were distributed in low (1), medium (1) and high (2) altitudes and they have a mean number of taxa below average (23.5 ± 2.9 SE). Navicula madeirensis occurred in association with Planothidium frequentissimum (Lange-Bertalot) Lange-Bertalot, Rhoicosphenia abbreviata (C.Agardh) Lange-Bertalot and Karayevia clevei (Grunow) Round & Bukhtiyarova, apart from the ubiquitous Achnanthidium minutissimum, Amphora pediculus, Cocconeis placentula var. euglypta and Planothidium lanceolatum.

In this survey, 60 records were new, not only to Madeira Island, but also to the Madeira Archipelago (Table 3). These include 55 species, two varieties and three genera (sp.).

Table 3.

New records for Madeira Archipelago and respective sampling sites.

Class Order Family First records for Madeira Archipelago Sampling sites
Bacillariophyceae Bacillariales Bacillariaceae Nitzschia acidoclinata Lange-Bertalot 1976 MAD40
Nitzschia alpinobacillum Lange-Bertalot 1993 MAD40
Nitzschia clausii Hantzsch 1860 MAD25
Nitzschia dealpina Lange-Bertalot & Hoffmann 1993 MAD28
Nitzschia filiformis (W.Smith) Van Heurck 1896 MAD14
Nitzschia filiformis var. conferta (P.G.Richter) Lange-Bertalot 1987 MAD14
Nitzschia fonticola (Grunow) Grunow 1881 MAD06; MAD11; MAD12; MAD13; MAD14; MAD31; MAD34; MAD36; MAD37; MAD40
Nitzschia perminuta Grunow 1881 MAD11; MAD12; MAD14; MAD16; MAD25; MAD34; MAD37; MAD38
Nitzschia pusilla Grunow 1862 MAD08; MAD09; MAD11; MAD21; MAD38
Nitzschia recta Hantzsch ex Rabenhorst 1862 MAD25; MAD26
Nitzschia tubicola Grunow 1880 MAD39
Cocconeidales Achnanthidiaceae Achnanthidium gracillimum (F.Meister) Lange-Bertalot 2004 MAD22
Achnanthidium jackii Rabenhorst 1861 MAD30
Achnanthidium saprophilum (H.Kobayashi & Mayama) Round & Bukhtiyarova 1996 MAD32
Achnanthidium straubianum (Lange-Bertalot) Lange-Bertalot 1999 MAD01; MAD33; MAD36
Planothidium amphibium C.E.Wetzel, L.Ector & L.Pfister 2014 MAD09; MAD10; MAD12; MAD21
Planothidium daui (Foged) Lange-Bertalot 1999 MAD19
Planothidium dubium (Grunow) Round & Bukhtiyarova 1996 MAD40
Planothidium pumilum Bąk & Lange-Bertalot 2015 MAD01; MAD34; MAD36; MAD40
Psammothidium hustedtii (Krasske) S.Mayama 2002 MAD17; MAD19; MAD30; MAD35
Cymbellales Anomoeoneidaceae Adlafia bryophila (J.B.Petersen) Lange-Bertalot 1998 MAD14; MAD15; MAD21; MAD22; MAD23; MAD36
Adlafia minuscula (Grunow) Lange-Bertalot 1999 MAD21
Adlafia multnomahii E.A.Morales & M.Lee 2005 MAD14
Gomphonemataceae Encyonema amanianum Krammer 1997 MAD23
Gomphonema augur Ehrenberg 1841 MAD40
Gomphonema clavatulum E.Reichardt 1999 MAD07; MAD08; MAD11; MAD14; MAD15; MAD23; MAD25; MAD26; MAD27; MAD31
Gomphonema minutum (C.Agardh) C.Agardh 1831 MAD23; MAD40
Rhoicospheniaceae Gomphosphenia lingulatiformis (Lange-Bertalot & E.Reichardt) Lange-Bertalot 1991 MAD14; MAD15; MAD39
Eunotiales Eunotiaceae Eunotia arcus Ehrenberg 1837 MAD22
Eunotia sp. MAD08; MAD18; MAD19; MAD24; MAD27
Eunotia paludosa Grunow 1862 MAD31
Fragilariales Fragilariaceae Fragilaria microvaucheriae C.E.Wetzel & Ector 2015 MAD26; MAD35; MAD36; MAD37; MAD40
Fragilaria pectinalis (O.F.Müller) Lyngbye 1819 MAD03; MAD21; MAD23; MAD26; MAD36; MAD40
Fragilaria perminuta (Grunow) Lange-Bertalot 2000 MAD16; MAD25
Fragilaria recapitellata Lange-Bertalot & Metzeltin 2009 MAD06; MAD12; MAD31; MAD32; MAD34; MAD38
Fragilaria rumpens (Kützing) G.W.F.Carlson 1913 MAD05; MAD20; MAD22; MAD23; MAD28; MAD30; MAD35
Staurosiraceae Pseudostaurosira elliptica (Schumann) Edlund, Morales & Spaulding 2006 MAD08
Pseudostaurosira robusta (Fusey) D.M.Williams & Round 1988 MAD30
Pseudostaurosira subconstricta (Grunow) Kulikovskiy & Genkal 2011 MAD26
Staurosirella lapponica (Grunow) D.M.Williams & Round 1987 MAD23
Mastogloiales Achnanthaceae Achnanthes brevipes var. brevipes Agardh 1824 MAD40
Achnanthes minuscula Hustedt 1945 MAD40
Naviculales Diadesmidaceae Humidophila brekkaensis (Petersen) R.L.Lowe, Kociolek, J.R.Johansen, Van de Vijver, Lange-Bertalot & Kopalová 2014 MAD27
Diploneidaceae Diploneis praetermissa Lange-Bertalot & A.Fuhrmann 2016 MAD19
Naviculaceae Caloneis silicula (Ehrenberg) Cleve 1894 MAD34
Hippodonta capitata (Ehrenberg) Lange-Bertalot, Metzeltin & Witkowski 1996 MAD02; MAD14
Navicula angusta Grunow 1860 MAD13; MAD14; MAD40
Navicula cryptotenelloides Lange-Bertalot 1993 MAD40
Navicula recens (Lange-Bertalot) Lange-Bertalot 1985 MAD26; MAD29
Navicula rostellata Kützing 1844 MAD28; MAD29; MAD40
Navicula tantula Hustedt 1943 MAD07; MAD08; MAD14; MAD25; MAD26, MAD27, MAD28; MAD33; MAD34; MAD37; MAD39
Navicula tenelloides Hustedt 1937 MAD14; MAD 25; MAD27; MAD34
Sellaphoraceae Sellaphora atomoides (Grunow) Wetzel & Van de Vijver 2015 MAD02; MAD04; MAD07; MAD08; MAD12; MAD13; MAD15; MAD16; MAD17; MAD18; MAD21; MAD22; MAD23; MAD29; MAD30; MAD33; MAD34; MAD36; MAD38; MAD39; MAD40
Stauroneidaceae Craticula molestiformis (Hustedt) Mayama 1999 MAD11; MAD12; MAD30
Fistulifera saprophila (Lange-Bertalot & Bonik) Lange-Bertalot 1997 MAD01; MAD02; MAD04; MAD09; MAD10; MAD12; MAD13; MAD14; MAD15; MAD16; MAD17; MAD18; MAD20; MAD21; MAD33; MAD34; MAD35; MAD38; MAD40
Surirellales Surirellaceae Surirella terricola Lange-Bertalot & E.Alles 1996 MAD40
Thalassiophysales Catenulaceae Amphora sp. MAD31
Coscinodiscophyceae Aulacoseirales Aulacoseiraceae Aulacoseira sp. MAD04
Mediophyceae Stephanodiscales Stephanodiscaceae Stephanodiscus minutulus (Kützing) Cleve & Möller 1882 MAD14
Stephanodiscus parvus Stoermer & Håkansson 1984 MAD40

Discussion

The diatom diversity (130 taxa belonging to 44 genera) displayed by Madeira Island in the 27 permanent streams in this study is due to the habitat complexity (including water quality, habitat structure and climate), as well as large scale-effects stemming from the Islands’ isolation and geographical location as was found on other oceanic islands (Flower 2005, Gonçalves et al. 2015).

Diatom diversity from Madeira Island is relatively low when compared to other oceanic islands and continental regions (e.g. Herlory et al. 2013, Gonçalves et al. 2015, Jyrkänkallio-Mikkola et al. 2018). This kind of comparison is difficult to make since it depends on the sampling efforts: the number of samples analysed, the timing of the samplings, the number of surveys carried out, the physical and chemical composition of the waters, the number of substrates sampled and the taxonomic effort with which the diatom valves were analysed (Morales et al. 2009, Veselá and Johansen 2009).

Nonetheless, when comparing archipelagos in the Macaronesia Region, 201 diatom taxa were recorded in 316 samples from 14 permanent Azorean streams (Gonçalves et al. 2015). However, the number of diatom taxa recorded per sample was higher in Madeira Island (24.1 ± 1.1 SE) than in the Azores (20.9 ± 2.7 SE). Additionally, it is worth mentioning that this study is focused on the main island from the Madeira Archipelago which does not represent the different microhabitats present in all the other islands.

Comparisons to other regions in the world reveal how diatom diversity in Madeira Island is “poor”. For instance, in a tropic region (Sub-Saharan Africa), the number of diatom taxa identified in 67 sites in Kenya was significantly greater (297 taxa) than the number of taxa recorded in Madeira Island (Jyrkänkallio-Mikkola et al. 2018).

The low diversity of freshwater biota has already been reported to the Madeira Archipelago (Hughes and Malmqvist 2005), as well as other oceanic islands in the world (Brasher et al. 2004, Flower 2005, Delgado et al. 2012).

This insular oceanic ecosystem should offer some degree of isolation from continental floras, but the special conditions that promote speciation on islands are not present, for example, extreme water quality or geological age and activity (Flower 2005). It is therefore unsurprising that the great majority of taxa had a cosmopolitan distribution (e.g. Achnanthidium minutissimum, Planothidium lanceolatum, Nitzschia soratensis, Cocconeis placentula var. euglypta) (Flower 2005, Gonçalves et al. 2015). This indicates a lack of isolation mechanisms operating in the Island, but the diatom taxa present are nevertheless clearly attributable to different biogeographical regions.

The Macaronesia Region should register more endemisms for many groups as do other regions (e.g. Antarctic Region, Van de Vijver et al. 2011, Van De Vijver et al. 2004, Kopalová et al. 2009, Kopalová et al. 2011), but due to the lack of research in this Region as stated before, only a few have been found. This might be one of the reasons why we had some taxonomic challenges. Diatoms are extremely diverse and there are many species that have not been described yet, thus the species delimitation is still controversial (Licea et al. 2016). For some diatom groups, it was difficult to distinguish closely-related species because of their wide morphological variation or because many taxa frequently differed in detail from published descriptions. We took a conservative attitude for these taxonomic differences and disregarded small differences in morphometric data.

Considering the significant differences between the islands of Madeira Archipelago, such as geological ages, volcanic composition, climate patterns and distribution, land uses, types of forest and orography, we expect higher diatom species richness and exclusive taxa from these islands (Porto Santo, Desertas, Selvagens). Furthermore, increasing the sampling effort in Madeira Island, for instance, by sampling other streams and/or other sites in the Laurissilva forest, may result in the identification of other diatom taxa. Additionally, higher time replication and larger datasets are required to better understand distribution patterns and large-scale spatial patterns of species dispersal. According to Smucker and Vis (2011), there is a significant under-estimation of diatom diversity when exclusively collecting epilithic habitats for documenting species distribution and for conservation purposes. Taking this into consideration, different methods to obtain the greater number of diatom taxa for each site are encouraged, for instance, by following two distinct sampling techniques (transects located in riffles and from microhabitats) (Falasco et al. 2016).

The factors controlling taxa richness, as well as the regional endemic taxa, remain unclear as there is no convincing link with the simplified habitat features recorded during the study and more research is needed. Relationships are probably multivariate in nature and include site history, unmeasured micro-habitat availability and climate (Flower 2005, Pajunen et al. 2016).

The results of this study provide baseline knowledge on the current distribution of freshwater diatoms on Madeira Island streams, revealing a distinct, but taxonomically simple diatom flora, typical from oceanic island ecosystems (Flower 2005, Gonçalves et al. 2015). In order to better understand the complexity of these streams, depth studies on the temporal and spatial distribution patterns, population dynamics, species’ interactions, guilds and traits are essential for improving knowledge and the development of future effective monitoring and conservation programmes and measures for local stakeholders.

Acknowledgements

This work was funded by national funds through FCT – Foundation for Science and Technology under the PTDC/CTA-AMB/28511/2017 and DL57/2016/ ICETA/EEC2018/25. The field surveys comply with the current laws of Portugal. Thanks are also extended to the reviewers for their very useful comments on earlier versions of this manuscript.

Author contributions

CR, PMR, VG conceived the study and PMR and VG carried out the sampling campaign in Madeira Island. CR prepared and identified the epilithic diatoms and VG supervised. CR wrote the paper with inputs from all authors. All authors agree with the final version of the paper.

References