T HE OCCURRENCE OF NETHERLANDS HET SALVINIA NATANS IN THE DURING THE MIDDLE VOORKOMEN VAN SALVINIA HOLOCENE NATANS IN NEDERLAND HOLOCEEN GEDURENDE HET W A. O Leiden University, Faculty of Archaeology PO Box 9515, 2300 RA Leiden, the Netherlands waout@hotmail.com Abstract In 1966, Zandstra published a paper on the occurrence of Salvinia natans in Atlantic Rhine deposits in the Netherlands. Since then, research in Dutch wetlands has yielded several more subfossil finds. This paper aims to provide an updated overview of finds and to summarise the information on their origin, distribution and age. Samenvatting In 1966 verscheen er een artikel van Zandstra over het voorkomen van Salvinia natans in Atlantische Rijnafzettingen in Nederland. Er zijn sindsdien verscheidene nieuwe subfossiele vondsten aangetroffen in Nederlandse wetlands. Dit artikel beoogt een nieuw vondstoverzicht te geven en een samenvatting van de herkomst, verspreiding en ouderdom van de vondsten. Introduction Salvinia natans is only seldom found at Dutch archaeological sites and is presumably not part of the present Dutch vegetation. Subfossil finds are known from palaeoecological studies in Europe from Pleistocene interglacials, particularly from the Eemian (Ipswichian; e.g. Turner 2000; van der Ham et al. 2008), as well as from the Holocene (e.g. Zandstra 1966). During an archaeobotanical literature study on Late Mesolithic and Early and Middle Neolithic Dutch wetlands sites, it appeared that Salvinia natans has regularly been identified and reported. Wim Kuijper has been involved directly or indirectly in the research on many of these sites. Wim explained that colleagues initially raised the question whether the Dutch MidHolocene finds could possibly represent reworked Pleistocene material or material transported by the Rhine from more southern regions. Indeed, long-distance transport by river water is a relevant factor of pollen deposition in the river area (Florschütz and Jonker 1940; van der Woude 1983, p. 20-23). Therefore, this paper aims to provide a short overview of earlier research on Holocene finds of Salvinia natans and of more recently reported finds, mainly based on the analysis of published archaeobotanical literature. It is investigated what the indications of local occurrence of Salvinia natans are in the Netherlands during the Holocene and when and where the species occurred. out - salvinia natans in the netherlands during the holocene 205 Fig. 1 Development of Salvinia natans. Upper part ig­ ure: sporophyte; node with sporocarps; sporocarps with microsporangia (left) and megasporangia (right). Middle part igure: megasporangium with megaspore that is surrounded by the perispore; microsporangium with massulae containing mi­ crospores; megasporangium with prothallium and archegonia; microsporangium with prothallia and antheridia. Lower part igure: megasporangium, prothallium and stabilising leaves; megaspore, sta­ bilising leaves, prothallium and young sporophyte; (megaspore,) stabilising leaves, prothallium and sporophyte Illustration: Library University of Amsterdam (UvA), special coll., SAE 01-077.538 Ontwikkeling van Salvinia natans. Bovenste deel iguur: sporoiet; knoop met sporocarpen; sporo­ carpen met microsporangia (links) en megaspo­ rangia (rechts). Middelste deel iguur: megaspo­ rangium met megaspore die omringd is door de perispore; microsporangium met massulae die microsporen bevaten; megasporangium met pro­ thallium en archegonia; microsporangium met prothallia en antheridia. Onderste deel iguur: megasporangium, prothallium en stabiliserende bladeren; megaspore, stabiliserende bladeren, pro­ thallium en jonge sporoiet; (megaspore,) stabiliser­ ende bladeren, prothallium en sporoiet Ecology of Salvinia natans Salvinia natans (Salviniaceae) is an annual, free-floating aquatic fern with leaves of up to 15 cm long (see Figure 1). The stem and leaves are covered with small hairs (van der Meijden 1996). The species has an Eurasian, continental distribution. At present, it occasionally occurs in the Netherlands but this presumably concerns adventives only. The species grows in fresh, eutrophic, stagnant water of 2-4 metres depth (Zandstra 1966; Wolff and Schwarzer 2005). The species is thermophilous and needs a water temperature of 14-17°C for germina- 206 of plants and snails tion (Wolff and Schwarzer 2005). Reproduction occurs vegetatively and by spores. The species is heterosporous and produces trilete microspores that function as male gametophytes and trilete megaspores that function as female gametophytes. Earlier research Florschütz and Jonker (1940) published the first Dutch or even first West European Holocene finds of Salvinia natans from Utrecht and Wijk bij Duurstede. The authors excluded the possibility of transport of Salvinia natans from the upper Rhine regions because of the presence of intact megasporangia (characterised by a reticulate network), the occurrence of remains in several samples at a specific depth only and the shape of the Salvinia curve in the published diagram that points to local presence (Florschütz and Jonker 1940). In 1966, Zandstra presented the three Dutch Holocene Salvinia find spots that were known at that time (Alphen aan de Rijn, Utrecht and Wijk bij Duurstede, all dating to the Atlantic) and additionally presented a pollen diagram from a new find location, the Vuylcop polder (province of Utrecht). Zandstra provided a reconstruction of the environment of Salvinia natans, listed the water plants that were found together with Salvinia natans, and also argued that the Salvinia finds from Vuylcop were of local origin. Arguments in favour of a local origin are the finds of microspores, megaspores and many massulae (containing mature spores), the contemporaneous presence of other aquatics, and the occurrence of Salvinia natans over a considerable period that is restricted to the Atlantic. The paper appears to suggest that Salvinia natans is an indicator species of the Atlantic (Zandstra 1966, p. 392-393; further discussed below). The diagram of Vuylcop (not dated) also shows Salvinia finds in the Sub-Boreal pollen zone; these finds are not discussed. In the diagram of Alphen aan de Rijn (Jelgersma 1961), Salvinia microsporangia are present in eight subsequent samples (one of them dating to 5306-4851 cal BC), and the values reach up to 3%. Jelgersma also identified microspores of cf. Salvinia (quantity unknown) in a clay sample from a core at Ternaard (province of Friesland) that is slightly older than 3341-2580 cal BC (Sub-Boreal). Local occurrence of S. natans at this location is unlikely since it concerns a single sample from sediment that does not exclude transport of the spore from elsewhere. Moreover Jelgersma interpreted the relevant pollen zone as indicative of a brackish, marine environment while S. natans is a freshwater plant. Results: new finds of Salvinia natans During a recent archaeobotanical literature study, it became clear that various new Dutch subfossil finds of Salvinia natans have been documented since the publication by Zandstra. Table 1 shows the names of all Dutch Holocene finds that are known to the author, the type of finds, and references. Figure 2 shows the localities. All sites are discussed below, as well as some details of the relevant samples (see Out 2009 for more information on most archaeological sites). The finds are dated indirectly by stratigraphy or by interpretation of the pollen spectra. The overview may be incomplete due to an underrepresentation of geological and palaeoecological sources. Table 2 shows which other indicators of open water were found in samples that contained Salvinia natans (non-pollen palynomorphs not included). For individual sites, taxa from various samples are combined. Taxa indicative of other vegetation types, such as woodland of dry terrain, alder carr and marshes, are not provided. Table 2 shows that remains of out - salvinia natans in the netherlands during the holocene 207 site 1 Wijk bij Duurstede 2 Utrecht (Domplein) 3 Alphen aan de Rijn 4 microspores megaspores + + Florschütz and Jonker 1940 reference 1 Florschütz and Jonker 1940 + Jelgersma 1961 Ternaard cf. + Jelgersma 1961 5 Polder van Vuylkop ++ 6 Goudriaan + Out 2009 7 Bergambacht + Out 2009 8 Rotterdam Central Station 1 Guiran and Brinkkemper 2007 9 Hardinxveld-Giessendam Polderweg ++ Bakels and Van Beurden 2001 9 Hardinxveld-Giessendam De Bruin + Bakels et al. 2001 10 Bergschenhoek 1 Out 2009 11 Meerdonk 12 Brandwijk-Kerkhof 13 Hazendonk 14 Hornaar-Lage Giessen 15 Schokkerhaven-E170 (cf.) + ++ + Out 2009; Verbruggen in prep. ++ Out 2008; 2009 ? ++ Table 1 Holocene inds and ind locations of Salvinia natans in the Netherlands. The numbers of the site loca­ tions correspond with the numbers in Fig. 1. The distinction between spores and sporangia is not always clear from the literature sources; the text provides more information on some of the inds. 1: one ind, +: a few, ++: many Zandstra 1966 Out 2009 + Van Hoof et al. 2008 ++ Weijdema et al. in prep. Holocene vondsten en vindplaatsen van Salvinia natans in Nederland. De nummers van de vind­ plaatsen komen overeen met de nummers in Fig. 1. Het onderscheid tussen sporen en sporangia is niet altijd duidelijk in de literatuur; de tekst geeft meer informatie over sommige vondsten. 1: één vondst, +: enkele, ++: veel Chara, Nuphar lutea, Nymphaea alba and Potamogeton are most commonly found together with S. natans in palaeoecological and archaeobotanical samples. Almost all water plants that are found together with S. natans are indicative of eutrophic conditions and predominantly indicative of stagnant to slowly running water. This confirms the earlier conclusions of Zandstra (1966). Many of the taxa tolerate weak brackish conditions, but none needs brackish or marine conditions, while the common species Nuphar lutea and Nymphaea alba do not tolerate such conditions at all. Bergambacht An undated pollen diagram of Bergambacht was analysed in 1974 as part of a study by the Stichting voor Bodemkartering (archive K. Koelbloed). Microspores of Salvinia (not quantified) were attested in three samples in the first zone of the diagram that presumably dates to the Late Atlantic and/or Sub-Boreal (based on interpretation of the presence of various taxa including Salvinia). The sediment of the relevant part of the core consisted of peat with wood remains. Although there is only little context information, the find seems to represent local vegetation as the age and the sediment do not suggest the presence of reworked material. 208 of plants and snails 4 15 3 2 8 10 7 12 11 14 Palaeogeography: coastal barriers tidal flats 5 1 6 13 9 salt marshes Pleistocene coversand river deposits inland dunes fen peat Salvinia natans raised bogs cf. Salvinia natans Fig. 2 Find locations of Salvinia natans. See Table 1 for the names of the locations 0 50km Vindplaatsen van Salvinia natans. Zie Tabel 1 voor de plaatsnamen Goudriaan The pollen diagram of Goudriaan resulted from research by the National Geological Survey (de Jong 1985). Two samples in the highest zone of the diagram, dating after 2900 cal BC and characterised by clayey sediment, contained microspores (not quantified) of Salvinia. Due to the type of sediment and the age, it is not clear whether it concerns local vegetation out - salvinia natans in the netherlands during the holocene 209 De Bruin, phase 3 Bergschenhoek Meerdonk Brandwijk-Kerkhof Hazendonk m m p/m p m p/m p salinity max Polderweg, phase 2 p salinity min Polderweg, phase 1 m water current ext Bergambacht m water current max Rotterdam CS p water current min Wijk bij Duurstede p ecological group Vuylcop type of remains characteristics taxon Alphen a/d Rijn site 4a 9 9 9 0 0-2 4a 9 9 9 0 1-2 4b 9 9 9 0 0 4a 1 2 0 0 1 4a 1 2/1 3/2 0 0/2 0 0/1 Taxon and type of remains Callitriche sp., pollen + Callitriche sp., macroremains Ceratophyllum sp., pollen + + + Ceratophyllum sp., macroremains + Chara sp., oospores + + + + Elatine hydropiper, macroremains + + Elatine triandra, macroremains + + + Eupotamogeton, pollen + Hydrocharis morsus-ranae, macroremains + Lemna sp., pollen + Myriophyllum sp., pollen + + Myriophyllum spicatum, pollen + 4a 1 5 0 0 1 Myriophyllum verticillatum, pollen + 4b 1 2 0 0 0 + 4a 1 2 0 0 2 + 4a 1 2 0 0 0 4a 1 2 3 0 0 + 4a 1 1 2 0 0 + 4a/b 1 2/3 0 0-2 4a 1 3 0 0 2 4a 1 3 0 0 2 4a 1/2 5/2 0 1 + 1 1 9 0 0 + 1 2 0 0-1 2 1 2 0 0 2 cf. Myriophyllum verticillatum, macroremains + Najas marina, macroremains + Najas minor, macroremains + Nitella sp./Nitellopsis sp., oospores + Nuphar sp., pollen + + + + Nuphar lutea, macroremains Nymphaea sp., pollen Potamogeton sp., macroremains + + Nymphaea alba, macroremains Potamogeton sp., pollen + + + + + + + + + + + + + + + + + + + + + + + + Potamogeton cf. pectinatus, pollen + Potamogeton perfoliatus, macroremains Ranunculus aquatilis-type, macroremains Trapa natans, spines + + + + Zannichellia palustris, macroremains Z. palustris ssp. pedicellata, macroremains 210 + of plants and snails + 4a Table 2 (left page) Plants and green algae that were found as pollen or botanical macroremains in samples that also contained Salvinia natans, and some characteristics of those species (Biobase 2003). Details of the site Schokkerhaven-E170 are not included in this table. Type of remains: p: pollen; m: botanical macrore­ mains. Ecological group: 4a: plants of eutrophic water; 4b: plants of oligotrophic water. Water current: 0: unknown; 1: stagnant; 2: sluggish; 3: steady; 5: very quickly running; 9: not relevant. Salinity: 0: fresh; 1: weakly brackish; 2: brackish. Min: minimum; max: maximum; ext: extreme. +: present Planten en groene algen waarvan pollen of bota­ nische macroresten zijn gevonden in monsters die ook Salvinia natans bevaten, en enige kenmerken van die soorten (Biobase 2003). Details van de vindplaats Schokkerhaven-E170 zijn niet in deze tabel opgenomen. Type resten: p: pollen; m: bota­ nische macroresten. Ecologische groep: 4a: planten uit eutroof water; 4b: planten uit oligotroof water. Stroomsterkte van het water: 0: onbekend; 1: stil­ staand; 2: traag; 3: constant; 5: heel snel stromend; 9: niet relevant. Zoutgehalte: 0: zoet; 1: zwak brak; 2: brak. Min: minimum; max: maximum; ext: ex­ treem. +: aanwezig or reworked material, while other pollen curves suggest the possibility of inter-regional pollen transport by river water (Out 2009, p. 41). The relevant sample did not contain other remains of aquatics. Rotterdam Central Station A gully next to the site of Rotterdam Central Station yielded one megaspore of S. natans (Guiran and Brinkkemper 2007). The spore, collected from a clayey core sample, dates between 5600 and 5400 cal BC (van Haaster and Brinkkemper 1995; RADAR version 2006). The sample that contained Salvinia natans also contained indicators of brackish conditions, which were presumably transported by water from salt marshes located west of the site (Guiran and Brinkkemper 2007, p. 38-42). Hardinxveld-Giessendam Polderweg Megaspores and megasporangia of Salvinia natans from Hardinxveld-Giessendam Polderweg date to the occupation phases 1 (5500-5300 cal BC) and 2 (around 5000 cal BC) (Bakels and Van Beurden 2001; Louwe Kooijmans 2001a). Finds were retrieved from samples consisting of clayey peat, sandy peat, peat and peaty clay. Salvinia natans was found in c. 30 samples. The estimated quantification (resulting from counting fractions of samples) in individual samples varies between one and ten thousand. Hardinxveld-Giessendam De Bruin Megaspores of Salvinia natans from Hardinxveld-Giessendam De Bruin are known from three samples, all dating to the third occupation phase (4700-4450 cal BC) (Bakels et al. 2001; Louwe Kooijmans 2001b). The sediment of the relevant samples consisted of sandy peat, clayey peat and peaty clay. In contrast to Polderweg, only several dozens of spores were found. Bergschenhoek People visited Bergschenhoek seasonally during 10-20 years between 4350 and 4050 cal BC (Louwe Kooijmans 1987). The humic clay directly next to the site and presumably contemporaneous with occupation revealed one megaspore of Salvinia natans (identification W.J. out - salvinia natans in the netherlands during the holocene 211 Kuijper; Out 2009). The sample that contained the megaspore contained also, apart from remains of other freshwater taxa, a pollen grain of Armeria/Limonium, indicating brackish influxes. Meerdonk A few megaspores of Salvinia natans were found in a peaty core sample related to occupation at the river dune Meerdonk between 4030-3910 cal BC (Verbruggen in prep.). The botanical remains were identified by W.J. Kuijper. Brandwijk-Kerkhof At Brandwijk-Kerkhof, megaspores of Salvinia natans were attested in peaty and clayey layers around the river dune dating from 4470 to 3630 cal BC (layers 45, 50, 60 and 70) (identification W.J. Kuijper; Out 2009; Verbruggen in prep.). Salvinia natans was particularly found in those samples sieved on a 0.25 mm sieve, including most of the samples of two sample columns. Salvinia remains were common in the best investigated layer 50 (dating between 4220-3940 cal BC, consisting of sandy peat, estimated quantity of S. natans hundreds to a few thousands in four single samples) and a clay layer directly above layer 50, layer 70 (dating between 3700–3630 cal BC, estimated quantity of S. natans a few hundred megaspores). Many of the finds from layer 70 still included the fragile, reticulate sporangium wall, demonstrating good preservation and pointing to local growth (unpublished data W.J. Kuijper). Microspores were attested during pollen analysis (core D, sample depth: 521, 541 and 581 cm –NAP). The relevant samples are contemporaneous with the archaeological layers 50 and/or 60 (Out 2008). Hazendonk At the Hazendonk, occupied intermittently between 4000 and 2500 cal BC and located in the same area as various of the sites presented above, there are no finds of megaspores of Salvinia natans. This may be related to the local environmental conditions or possibly to the time of research, as it was one of the earliest investigated Stone Age river dunes in the local area. Finds of both pollen and carbonised macroremains of the thermophilous species Trapa natans dating to phase Haz 3 at this site (3670-3610 cal BC) suggest that the temperature was probably no restriction for the occurrence of Salvinia natans at the Hazendonk. The high quality analysis of pollen and non-pollen palynomorphs of the Hazendonk resulted in repetitive identification of trilete, psilate spores that were tentatively identified as cf. Salvinia natans (A. Louwe Kooijmans unpublished data). If not representing Salvinia, they could alternatively represent Azolla. The spores were regularly found in three different cores and sample columns (57, 3, and 2) and show high values (peak values up to 10% of the upland pollen sum) from phase Haz 2 onwards until phase Haz 3. The sediment of these samples consists of peat, clayey peat and clay, and the pollen curves indicate an increased water level or an increase of the flooding frequency. Other pollen analyses at the Hazendonk (Louwe Kooijmans 1974; van der Wiel 1982, van der Woude 1983) did not demonstrate the presence of Salvinia natans or Azolla. 212 of plants and snails Hoornaar-Lage Giessen The site Hoornaar-Lage Giessen was occupied somewhere between 4000 and 3700 cal BC (van Hoof et al. 2008). The botanical remains of a single sample from the find layer, consisting of 5 litres of humic sand rich in charcoal, were poorly preserved but nevertheless contained megaspores of Salvinia natans (identification W.J. Kuijper). The sample did not contain other aquatics. Schokkerhaven-E170 Analysis of a samples series at Schokkerhaven-E170 revealed microspores, microsporangia and megasporangia of Salvinia natans, dating between c. 4000 and 3700 cal BC (Weijdema et al. in prep., second opinion S. natans by W.J. Kuijper; pers. comm. O. Brinkkemper). Discussion Local occurrence of S. natans in the Netherlands during the Holocene This overview shows that various new finds of Salvinia natans in the Rhine/Meuse river area have been discovered since the 1960s. As the presented results are based on a study that focused on certain regions and types of sources, it is expected that there are more find locations. As already concluded by Florschütz and Jonker (1940) and Zandstra (1966), the new results confirm that Salvinia natans occurred in Dutch wetlands during the middle Holocene. The species has been found at a considerable number of sites and has also been found at some individual sites (Polderweg, Brandwijk-Kerkhof, and Schokkerhaven-E170) in a considerable number of samples. Additionally, the finds have been derived from deposits consisting of peat, sandy or clayey peat and clay. In the case of finds from clay, long-distance transport could be the case, but this is less likely for samples from peaty sediment. The megaspores at the find locations Polderweg, Brandwijk-Kerkhof and Schokkerhaven-E170 were still covered by the reticulate sporangium wall, which would be less likely in the case of long-distance transport or reworking of material. (The presence of megasporangia is not always clear for other sites where megaspores have been found). At Brandwijk-Kerkhof and Schokkerhaven-E170, both micro- and megaspores have been identified. One sample series from Brandwijk-Kerkhof furthermore shows a curve of Salvinia natans that supports the (extra-)local growth of the species. Nevertheless, local occurrence of S. natans is not demonstrated for all of the sites, particularly not for Ternaard, Rotterdam Central Station, Goudriaan and Bergschenhoek, as these finds may represent single finds, transported by river water from elsewhere in the Dutch river area. Local occurrence is particularly unlikely for the find from Ternaard (see the section Results). Former distribution in the Netherlands during the Holocene Almost all Dutch Holocene finds of Salvinia natans are distributed from Rotterdam in the west to Wijk bij Duurstede in the east. Interestingly, the find location at SchokkerhavenE170 (province of Flevoland) demonstrates that the occurrence Salvinia natans is not restricted to the Rhine/Meuse river area only but also occurred in more northern parts of the Netherlands. Furthermore, the two single finds at Rotterdam and Bergschenhoek show that remains of the species can also be found in the western part of the river area where marine influence occurred. Local occurrence of S. natans at these sites is, however, not demonstrated out - salvinia natans in the netherlands during the holocene 213 (see previous section). Furthermore, these finds do not imply that Salvinia natans tolerates brackish water since the local and contemporaneous occurrence of both the halophilous taxa and Salvinia natans is not demonstrated. Age The Salvinia records of Florschütz and Jonker (1940), Jelgersma (1961) and Zandstra (1966) all date to the Atlantic and were found in Rhine deposits. While Florschütz and Jonker (1940) stated that the finds are not necessarily restricted to the Atlantic, Zandstra (1966, p. 389, 393) suggested that it “would … appear likely that Salvinia natans had a natural habitat in the Rhine delta in the Atlantic”, and argued that “The Atlantic supplied the need of a warmer climate than that prevailing today” (despite Salvinia finds in a Sub-Boreal pollen zone). However, the youngest finds from Schokkerhaven-E170, Brandwijk-Kerkhof and Hoornaar-Lage Giessen date between 4000 and 3700/3630 cal BC, the transition from the Atlantic to the Sub-Boreal (cf. Weijdema et al. in prep.; see also Kuijper in van Hoof et al. 2008). The broad time range of most of the relevant deposits does not allow to draw a final conclusion on whether the species was present in the Netherlands during the Sub-Boreal. A single sample from Brandwijk-Kerkhof (layer 70) is nevertheless dated indirectly to the SubBoreal properly. Acknowledgements The author is grateful to Wim for the years of training in Archaeobotany, for sharing his knowledge and skills (also during the writing of this paper), and for the good atmosphere in the lab. The author would also like to thank O. Brinkkemper, who kindly provided suggestions including information on Schokkerhaven-E170, and M. Field for discussion of figure 1. References Bakels CC, Beurden LM van (2001) Archeobotanie. In: Louwe Kooijmans LP (ed.) Archeologie in de Betuweroute. Hardinxveld-Giessendam Polderweg. Rapportage Archeologische Monumentenzorg 83: 325-378 Bakels CC, Beurden LM van, Vernimmen TJJ (2001) Archeobotanie. In: Louwe Kooijmans LP (ed.) Archeologie in de Betuweroute. Hardinxveld-Giessendam De Bruin. Rapportage Archeologische Monumentenzorg 88: 369-434 Florschütz F, Jonker FP (1940) A botanical analysis of a Late-Pleistocene and Holocene profile in the Rhine delta. 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