Rotifers and lower crustaceans from South-western Iceland

Background Iceland has high availability of freshwater, and it is rich in brackish and coastal aquatic bodies. However, knowledge on rotifers and meiobenthic and planktonic crustaceans inhabiting these habitats is lacking, and the inland aquatic fauna in Iceland is relatively understudied in comparison with the fauna of adjacent marine ecosystems. The majority of past research focused on larger lakes with the exception of one study on rotifers from the 1950s (Bartoš 1951) and two more recent studies on crustacean fauna of shallow freshwater bodies (Novichkova et al. 2014, Scher et al. 2000). Data are particularly scarce for the south-western part of the country.


Introduction
Iceland is one of the countries with the highest freshwater availability according to UNEP's Vital water graphics (http://www.eoearth.org/view/article/152861/). Additionally, being an island, it is rich in coastal brackish and saline aquatic habitats. However, little is known about the microcrustaceans and rotifers inhabiting these numerous habitats. The freshwater fauna of Iceland is relatively understudied compared to the fauna of adjacent marine ecosystems. Exhaustive sampling of deep-sea fauna was conducted within the inter-Nordic BIOICE project. As a result, Apostolov (2011) recorded 32 copepod harpacticoids of which 20 are new for the fauna of Iceland.
The first data on freshwater microinvertebrate fauna of Iceland date back to the 19 century (Guerne and Richard 1892a, Guerne and Richard 1892b). The first study on the rotifer fauna from the middle of the 20 century listed 59 species or subspecies (Bartoš 1951). The majority of the available studies on inland water bodies focused on large lakes: Mývatn in the north-east (Örnólfsd and Einarsson 2004, Adalsteinsson 1979, Jónasson 1979, Lindegaard 1979; and Thingvallavatn (Antonsson 1992) and Kerið Lakes  in the south-west of the country. Recently scientists have become increasingly interested in the inland freshwater copepods and cladocerans from small freshwater bodies (Novichkova et al. 2014, Scher et al. 2000. Data on observed morphological variability and teratology of lower crustacean in subpolar environments, including Iceland, were presented by Sinev et al. (2012), Evtimova (2009), Pandourski andEvtimova (2006), Pandourski and Evtimova (2005). These aberrations affected the fifth pair of legs in calanoids, the posterior part of the body in cyclopoids, or the head and antennule in cladocerans.
Our study presents data on taxa composition of Rotifera, Cladocera, and Copepoda in various aquatic habitats from South-western Iceland, including marine interstitial, wet bryophytes, springs, brackish and freshwater ponds and lakes. th th

Materials and Methods
Samples were collected from various aquatic habitats from South-western Iceland. The sampling sites included marine interstitial habitat, puddles, swamps, freshwater or brackish lakes (Table 1, Fig. 1). Rotifers and lower crustaceans were collected using a qualitative plankton net (type "Apstein", mesh size 38 µm) and a hand-held plankton net (mesh size 40 µm). The hand-held plankton net was used for sieving the sand and rinsing the bryophytes in order to collect the invertebrates inhabiting these substrata. The material was fixed in 70% ethanol. The specimens were mounted temporarily in a mixture of glycerin and ethanol and were identified to the lowest practicable level following Wallace and Snell (2010), Sørensen (2009), Segers (1995), Einsle (1993), Monchenko (1974), Manuylova (1964). Harpacticoids were identified by Dr Apostolov and presented in earlier works (Apostolov 2014, Apostolov 2007.

Results
A total of 39 taxa from Rotifera, Cladocera, and Copepoda were recorded from Southwestern Iceland during our study. The most diverse were the rotifers with 21 taxa belonging to nine families and two orders. We found 11 taxa of copepods which belonged to five families from three orders, and seven taxa of cladocerans from three families. Twelve associated invertebrate taxa were also found in our samples Table 2. Map of Iceland with sampling locations from 1 to 12. For site numbers please see Table 1.  Keratella quadrata (Müller, 1786) was recorded at five of the sampled localities, while the copepod Eucyclops serrulatus (Fischer, 1851) and the cladoceran Alona affinis (Leydig, 1860) were found at four and three of the sites, respectively. Twenty-eight taxa were recorded only at one of the 12 sampling locations. We recorded the highest diversity of rotifers and the lowest diversity of crustaceans from bryophytes near Öxaráfoss waterfall in Þingvellir National Park.

Discussion
We present data on rotifers and lower crustaceans from 12 aquatic habitats. For two of the stations (6 and 7), the lakes Sikið and Leirvogsvatn, we present the first data on zooplankton, and possibly also the first data for some of the smaller habitats (e.g. stations 3, 4, 5, 11). The majority of the recorded taxa either have a cosmopolitan distribution or are previously known from Iceland. For three of the recorded species we found no prevoius records in the available literature from Iceland: the rotifers Trichocerca cf. mucosa (Stokes, 1896) and T. vernalis (Hauer, 1936), and the copepod Cyclopina gracilis Claus, 1862. Rotifera dominated the sampled water bodies, followed by Copepoda and Cladocera. The most frequent taxon was the rotifer Keratella quadrata, previously recorded from Iceland by Bartoš (1951) . All of the recorded rotifer species have a cosmopolitan distribution.
Many of the cladoceran taxa we recorded are frequently found in the arctic region. Acroperus harpae (Baird, 1835) is typical for the littoral fauna of freshwater lakes from the Holoarctic region (Novichkova et al. 2014, Sinev et al. 2012 . We found these two species in permanent freshwater lakes (stations 6, 8, and 10).
All of the freshwater cyclopoid crustaceans recorded have cosmopolitan distribution and have been previously recorded from Iceland. We found only one marine copepod Cyclopina gracilis Claus, 1862. It is very common in the North Atlantic Ocean (Carey 1992, Grainger and Mohammed 1991, Mohammed and Neuhof 1985 but previously has not been reported from Iceland. The dominant cyclopoid in our samples was Eucyclops serrulatus (Fischer, 1851). Cyclops abyssorum Sars, 1863 is known to be among the dominant copepods in the large Icelandic lakes and is an important structural element for their zooplankton assemblages (Novichkova et al. 2014, Antonsson 1992. According to Larsen and Røen (1964) and Scher et al. (2000) another common cyclopoid for Iceland is Megacyclops viridis (Jurine, 1820). We found both C. abyssorum and M. viridis as well but only from shallow freshwater lakes (sites 6 and 8, correspondingly).
The two species of the harpactocoid genus Bryocamptus we recorded are associated with wet mosses , Apostolov 2007. Nitokra spinipes Boeck, 1865 can tolerate changes in salinity (Apostolov 2014) and was found from both brackish and freshwater habitats (sites 2 and 3).