The diversity of habitats and of the mutual connections between organisms and their environment can now be studied in even the most extreme ecosystems, including historical ecosystems no longer present. Moats were artificial bodies of water providing defence for inhabited strongholds in medieval times. Ecological studies of moats can therefore provide a window into historical influences of human activity on natural ecosystems. Ecological studies can be carried out on moats that have either been preserved from historical times (e.g. [1, 2]) or have been recently reconstructed. However, these modern moats do not perform the same function today as in the Middle Ages or Early Modern Period [3], and their aquatic ecosystems may therefore differ. The medieval stronghold’s inhabitants influenced water conditions by changing its trophic state, creating new habitats by placing construction elements, modifying its hydromorphology and sedimentation of defined types of water deposits, and their enrichment with selected metals (see [4, 5]). Therefore, the only way to get a clear window into those past ecosystems is through palaeoecological studies.
Geochemical studies of historical layers (such as core samples) conducted so far have in many cases confirmed their indicator role in environmental archeology [6]. The clearly higher Pb and Cu concentrations in the sediments of medieval moats have been used to determine the course of historical watercourses. However, the grain and mineral composition of sediments influences their susceptibility to metal sorption [7]. Therefore, geochemical mapping of archaeological sites, aiming to assess the distribution and characteristics of metallic pollutants in the environment, should take into account the location of the studied cores.
The chosen location of cores also influences paleoecological studies. Typically, paleoecological studies are based on one core of sediment, taken from the deepest (mostly central) part of the (palaeo)lake, on the assumption that the subfossil remains of the organisms once living in different parts of the water body are passively transported over time and deposited into the deepest area of the lake bottom [8]. This way, representative biotic records can be obtained with only one drilling, reducing cost and time of analyses. However, in the case of macrofossils, diatoms, or cladocerans, littoral taxa can be underrepresented in sequences derived from one core [9–11]. Analogously, chironomid head capsules accumulate mostly near the habitat of the larvae, and offshore transport is primarily observed in the less well preserved early instars [12], which may produce some biases. Generally, if the subfossil record is expected to adequately represent the environment of sedimentation and track particular events, multiple coring within the (palaeo)lake or mire is needed [8, 13].
In environmental archaeology, it is recommended to take at least two cores for palaeoecological analyses – one from the studied site (on-site profile) and one from an area separated from direct human activity (off-site) (e.g. [14, 15]). This allows examination of both local and regional vegetation history, and also of environmental information, such as past water level or temperature changes. Due to differences in deposition between aquatic and terrestrial systems, “wet sites” or “wet features” (such as a moat) allow examination with use of a wider range of ecological proxy analyses, based on organic deposits within cores sampled directly from archaeological trenches.
Our on-site palaeoecological investigations were an integral part of archaeological study of a stronghold’s moat system in Rozprza (central Poland; see ‘Study system’ section, below) [5, 16]. The main sediment core from the studied moat has already been investigated in detail (see [5]). The results indicated that when the stronghold was inhabited (14th -15th century AD), the moat was infilled with shallow water with possible episodes of rinsing through an artificial channel from the nearby river. Changes in water trophic state were tracked using chironomid-based models, and the Chironomidae-inferred summer temperatures of the Late Vistulian came from the nearby palaeomeander [17]. The Holocene chironomid sequence in the palaeomeander core was too low in abundance for quantitative reconstructions. However, even profiles with a depauperate biotic record can be used to obtain some ecological information, especially if treated simultaneously with other analyses. Therefore, we took additional cores from potentially different habitats (cf. [18]) within the moat, in order to obtain a more complete picture of environmental changes and human impact on the ecosystem in both time and space.
Though chironomids have proven to be useful in environmental archaeology (e.g. [19–23]), they have not previously been used for spatial reconstructions of artificial features. Moats, barays and other anthropogenic ponds have more limited potential to accumulate most of the subfossil remains in one point than lakes, mostly because of the specific basin morphology. Therefore, a carefully constructed spatial approach is required, taking into consideration habitat mosaics within the moat ecosystem. Chironomidae larvae represent an ideal proxy for spatial reconstruction of artificial features. They are ecologically diverse, sensitive to environmental changes, and indicative of particular ecosystem conditions and processes [24, 25]. Therefore, their distribution in shallow reservoirs with complex morphometry may be diversified and earlier studies based on one sediment profile [5] need to be complemented in order to cover the spatial aspect.
Considering these issues, our main goal was to reconstruct changes in habitat distribution across the stronghold’s moat system over time. We aimed to examine: 1) whether the Chironomidae communities were consistent in different parts of the moat, and 2) if they changed simultaneously and/or evenly throughout the moat basin. We hypothesised that even in such a small and shallow water body as a moat, the differences between cores would be significant and, thus, the habitat mosaic and more aspects of stronghold functioning could be reconstructed.
Study site
Rozprza is located in central Poland, about 60 km south of Łódź in the Piotrków Plain [26]. The study site is situated in the middle reach of the Luciąża River valley, a tributary of the Pilica River in the Vistula River basin (Fig. 1).
Nowadays, the late medieval stronghold remnants with their moat system are situated in an area covered by fields and meadows between the Rozprza and Łochyńsko villages (51˚18’07’’ N; 19˚40’04’’ E). The poorly preserved traces of moats, ramparts and baileys are however still visible in the field (Fig. 1C) and on DEMs (Digital Elevation Models). The study site is located on the valley floor with regulated channels of the Luciąża, Rajska and Bogdanówka rivers, as well as a dense network of drainage canals (Fig. 1B).
The studied fortress was established on the Plenivistulian fluvial terrace remnant, in the area of the widely spread valley floor. Such a location of the motte-and-bailey was reasonable for defensive reasons, as the sandy terrace remnant was protected by the surrounding swampy areas within the valley floor [16, 27]. However, the hillock of the terrace remnant occupied by the motte in the late Middle Ages was very low (up to 1 m).
Late medieval motte-and-bailey timber castle at Rozprza was built about 1330 AD and replaced an earlier timber and earth ringfort of unclear chronology (between 11th and 13th century AD). Motte-and-bailey castles were common in western Europe already in the 11th century [28] but introduced to Poland much later, in the 13th century [29]. In the 14th and 15th centuries AD it was one of the most popular types of rural noble residences.
The main moat of the Rozprza motte-and-bailey was established ca. 1330 AD and was later filled with organic (gyttja and peat) and partially inorganic deposits containing rich remains of wood (Fig. 2). The fill of the main moat was the subject of a detailed palaeoenvironmental study by Kittel et al. [5]. The accumulation of overbank silty sandy organic mud took place within the moat ditch system as late as in the 18th or 19th c. AD [5, 16]. The main moat had a width of 17–21 metres and a trapezoidal cross-section with a depth of 0.5 m, up to ca. 1.3 m. Wooden constructions were recorded near the inner slope of the moat ditch – in the form of a palisade created by two rows of vertical, sharpened wooden poles, and horizontal beams lying behind them. Those constructions were covered with thick layers of slope deposits (sand with organic mud). Many large chunks of wood (branches and boughs) were recorded in peat and organic mud of the upper unit of the moat fill close to the inner slope of the main moat [16].
The second moat had a width of about 11 m and a depth of ca. 0.5 m. In the trapezoid cross-section it has a flat bottom. The fill of the moat consisted of sand with organic admixtures at the very bottom, peat with silts, sands and organic mud, and also slope wash deposits on the moat slopes. The chronology of the feature formation has been estimated to 1485–1634 AD based on 14C data (Table 1). It demonstrates that the secondary moat was built probably in the early 16th century AD. However, the analysis of most archaeological small finds obtained during field works, mostly pottery fragments, estimated their age to the period between 14th and a mid-15th century AD. Evidence of human activity on the stronghold in the 16th century AD is also very limited. Therefore, an establishment of the second moat in the 15th century AD cannot be excluded (see [16]).
Table 1
The results of radiocarbon dating of the organic deposits of Rozprza moats
Core
|
Lithology
|
Depth
(cm b.g.l.)
|
Laboratory code
/dating method
|
Dated material
|
14C age (yr BP)
|
Calibrated age
(cal AD)
|
|
|
|
|
|
|
68.2%
|
95.4%
|
RP W3(2)
|
top of overbank organic mud
|
42–47
|
MKL-2839/LST
|
overbank deposits bulk
|
120 ± 40
|
1690–1922
|
1647–1943
|
top of peat
|
55–60
|
MKL-2840/LST
|
peat bulk
|
230 ± 35
|
1641–1800
|
1525-…
|
|
peat
|
75–80
|
MKL-2841/LST
|
peat bulk
|
320 ± 40
|
1506–1639
|
1472–1650
|
|
peat/coarse detritus gyttja
|
85–90
|
MKL-2842/LST
|
gyttja bulk
|
370 ± 35
|
1458–1622
|
1449–1635
|
|
upper part of coarse detritus gyttja
|
92–94
|
MKL-3504A/AMS
|
Rubus idaeus – 2 seeds
Solanum nigrum – 1 seed
|
409 ± 36
|
1440–1615
|
1428–1629
|
|
bottom of clayey coarse detritus gyttja
|
130–135
|
MKL-2843/LST
|
gyttja bulk
|
1280 ± 50*
|
667–773*
|
656–877*
|
|
sand with plant detritus
|
134–136
|
D-AMS 016324
/AMS
|
Rumex sp. – 2 inflorescences
Chenopodium rubrum – 1 leaf
Urtica dioica – 2 leaves
|
715 ± 43
|
1265–1380
|
1225–1390
|
RP W3(4)
|
top of overbank organic mud
|
38–43
|
MKL-2958/LST
|
overbank deposits bulk
|
100 ± 40
|
1694–1918
|
1680–1939
|
peat
|
50–55
|
MKL-2957/LST
|
peat bulk
|
200 ± 40
|
1655–1950
|
1642–1950
|
|
peat/coarse detritus gyttja
|
60–65
|
MKL-2956/LST
|
gyttja bulk
|
460 ± 40
|
1417–1455
|
1401–1616
|
|
bottom of clayey coarse detritus gyttja
|
75–80
|
MKL-2955/LST
|
gyttja bulk
|
1190 ± 60*
|
710–950*
|
681–990*
|
RP W1
|
bottom of muddy peat
|
20–22
|
MKL-A5577/AMS
|
Urtica dioica – 5 fruits
Mentha arvensis – 1 fruit
Thalictrum flavum – 1 fruit
Carex flava – 15 fruits
Sambucus sp. – 1 seed
|
479 ± 22
|
1425–1442
|
1414–1450
|
|
bottom of peat
|
40–45
|
MKL-2609/LST
|
peat bulk
|
610 ± 50
|
1305–1397
|
1289–1415
|
RP F2
|
bottom of sandy organic mud
|
38–43
|
MKL-2966/LST
|
organic deposits bulk
|
80 ± 40
|
1695–1916
|
1683–1936
|
|
bottom of peat
|
60–65
|
MKL-2967/LST
|
peat bulk
|
340 ± 50
|
1484–1634
|
1455–1646
|
* date recognised as outlier (redeposited material), not included in age-depth model |