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"RESEARCH ON SAND DUNE SYSTEMS AT OUED LAOU AND MAREMMA SITES"

 

Authors: Lorenzo Chelazzi, Isabella COlombini, Mario Fallaci, Elena Gagnarli (Partner 1)

 

Sandy beach arthropods at the two banks of the Oued Laou (Morocco)

The question addressed were: Are the physical and chemical characteristics of the substrate the same on the stretch of coast to the left and right of a river mouth?  Does beach morphology change according to the side of the river bank?  If so are arthropod populations the same or do they change in relation to the side of the river mouth inhabited?  How is species richness, diversity and abundance influenced by the environmental conditions found on the two sides of the river?

 

Stations 1 and 2 were chosen at the left side of the river, stations 3 and 4 at the right side; stations 2 and 3 were close to the river mouth. Spontaneously active arthropods were caught in pit fall traps made active for 72 hours. Beach profiles were measured at each transect; sand penetrability and other sand parameters were also measured (sand moisture, salinity, pH, total organic matter, grain size). Results were as follows. Beach profiles recorded in spring (April-May 2004) at the different stations showed that the steepest beach slope occurred at station 4 on the right side of the river mouth. Trends in beach slope were different on the two sides of the river mouth: steeper slopes were found farther away from the river mouth on the right side and closer to the river mouth on the left side. Beach profiles recorded in autumn of the following year (September-October 2005) substantially changed when compared to the previous year. Flatter profiles were recorded in all stations and again station 4 presented the steepest beach slope. In this case trends in beach slope were the same on both sides of the river mouth with steeper slopes at increasing distances from the river mouth.

In spring 2004 the total organic matter was evenly distributed in all stations with very similar values at the stations close to the river mouth. Higher mean values of pH were found at the stations on the left side of the river mouth and these became significantly higher from those of the right side. For these two parameters the same trends were observed in autumn 2005 with no significant differences with the previous sampling data. Regarding grain size analysis, in spring, finer sands (higher values of φ) were found at station 4 whereas around the mouth of the river coarser sands occurred, but no differences were found between these two stations. As the distance from the mouth increased there was an increase in the percentage of large pebbles on the beach surface. In autumn 2005 finer sands were again found at station 4 and in this case significant differences occurred between stations 2 and 3 with finer sand in the latter station. Furthermore, a smaller percentage of pebbles was recorded compared to the previous sampling period. Substrate conductivity never presented significant differences among stations with mean values of 272.9 and 242.9 μScm-1 respectively on the left and right side of the river mouth. In autumn 2005 station 2 presented a significantly lower mean value compared to the other stations. In spring 2004 the highest values of penetrability were found at stations 1 and 2 with a mean value of 44.9 mm when data were cumulated, showing that looser sands occurred on the left side of the river mouth. In autumn 2005 the situation changed and no significant differences were found on the two sides of the river even though for station 3 a higher penetrability was found compared to stations 1 and 2. In spring 2004 a total number of 6626 arthropods was caught during the three days of sampling. Of these 78 % was composed by the amphipod Talitrus saltator , 4.3 % by the isopod Tylos europaeus , and 13.08 % by the Coleoptera family of which 11.6 % was mainly represented by the adults and larvae of the tenebrionid Phaleria sp. Generally there was a linear increase of captures from station 1 to 4 and this was particularly true for the most abundant species (T. saltator , T. europaeus and Phaleria sp). The total number of arthropods captured at the stations on the right side of the Laou river mouth was six times the size of captures on the left side (n=900 at station 1 and 2 and n=5726 at station 3 and 4). Of the individuals captured on the two sides of the river mouth the crustacean species made up 85 % and 70.6 % of the total captures on the right and left side respectively, dominating the scenario. Also the adults of the tenebrionid Phaleria sp. were mainly found at the stations of the right side of the river mouth even if its larvae were more or less evenly distributed at the two sides. In autumn 2005 a lower number of arthropods (n=3677) were recorded during the three days of sampling. Again the amphipod Talitrus saltator was the most abundant species with 46.37 % of  captures, followed by the tenebrionid beetle Phaleria sp. with 40.82 % (considering both adults and larvae) and by the isopod Tylos europaeus with 7.94 % of captures. In this case station 3 presented the highest capture numbers in particular for T. saltator , T. europaeus , Labidura riparia and Phaleria sp, whereas station 4 for the Formicidae and the Thysanoptera. Again the stations on the right side of the Laou river mouth presented higher capture numbers compared to those on the left side (n=865 at stations 1 and 2, n=2812 at stations 3 and 4) confirming the patterns previously observed. Regarding species richness, in spring 2004 the highest number of species was found at station 3, where 31 different species were collected. Of these 16 were coleopteran species. At Oued Laou the highest values of α diversity calculated for the Coleoptera alone occurred around the river mouth with station 2 presenting the highest values. The composition of the Coleoptera community presented the highest values of evenness at station 2 and these corresponded to the lowest values of the dominance index. In autumn 2005 species richness was highest at station 4 in which a total of 16 species were found. Of these 7 belonged to the Coleoptera and reached their highest number proceeding from station 1 to 4. A total number of 9 species were collected on the right bank whereas only 3 on the left bank. Multiple regression analysis was carried out between the most abundant species (T. europaeus , T. saltator , Phaleria sp. and L. riparia ), the abundance, species richness, and α diversity of the Coleoptera community and the different environmental parameters (salinity, pH, organic matter, penetrability, slope, width, grain size of sand and of pebbles greater than 4mm). The results indicate that negative correlations generally occurred for salinity and pH values whereas positive ones were obtained for the other parameters except in the case of Coleoptera species and α diversity with slope and width.

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Ombrone river mouth diversity

The question addressed were: How do freshwater runoffs influence the community structure of terrestrial macroinvertebrates of sandy beaches? Are biological descriptors, such as species richness, diversity and abundance, influenced by increasing distances from freshwater discharges?  Do gradients of physical factors change at increasing distances from the river mouth? 

 

Five stations were chosen on the beach at the left side of the river Ombrone at increasing distances from the river mouth. Two transect were established from the base of the dune to the waterline and pitfall traps positioned for 72 hours to capture spontaneously active beach arthropods. Samples of sands were taken at each station to analyse: sand moisture, salinity, pH, total organic matter, grain size; sand penetrability, beach slope and width were measured on site (profiles of the beach). The samplings took place in September 2002 and May 2003. The results were as follows. Proceeding from the station closest to the delta to the ones farther away, there appears to be a clear trend both in the beach width and slope. At the first station beach slope was 2.7 % and 9.7 % in September 2002 and May 2003 respectively. This gradually decreased to 0.6 % and 1.6 % at the fifth station. Proceeding from the first station to the last, beach width varied from 12m to 38m and from 8m to 40m in autumn and spring respectively. In September the total organic matter of the sand varied between the highest mean value of 1.71 % at the first and second stations to the lowest mean value of 1.28 % at the forth one. In May slightly higher mean values of the total organic matter were recorded with the highest values in stations 1 and 5. Sand moisture had a mean value of 0.90 % at station 3 and of 3.67 % at station 5 during autumn. The latter value was significantly different from those registered at the other stations. In May mean values of sand moisture were more or less the same with station 5 the wettest one. Mean values of pH varied between 8.8 at station 2 and 9.3 at station 5 during autumn, whereas in spring pH values were slightly higher, especially at stations 1 and 2 where significant differences were registered. In the latter season there was a linear decrease from station 1 to station 4 with significantly higher mean values in the former station. Substrate conductivity presented highest values of 0.211 mScm-1 and 0.241 mScm-1, at station 1 and 5 respectively, whereas the lowest values of 0.114 and 0.113 mScm-1 were recorded at stations 3 and 4 respectively. In May mean values of sand conductivity were significantly higher than the previous season with values varying from 0.246 to 0.742 mScm-1 at station 3 and 5 respectively. Regarding grain size analysis finer sands (higher values of φ) were found with increasing distance from the river delta reaching greatest proportions at station 5 during both seasons. By contrast the highest values of penetrability were found at the first and second station in both seasons. In autumn 2002, a total number of 11354 arthropods was sampled on the eulittoral of which 98% was principally made up by the amphipod Talitrus saltator , the isopod Tylos europaeus , and the coleopteran tenebrionid Phaleria provincialis . The total number of arthropods captured at each station increased with the distance from the estuary. In particular this was true for T. saltator , the coleopteran crisomelid Gastroidea polygoni and the larvae of P. provincialis. Instead, for T. europaeus and the adults of P. provincialis, the greatest number of individuals were caught at station 4 and 3 respectively.  In spring 2003, a lower number (n=6430) of surface active arthropods were caught and again these were principally composed by T. saltator, T. europaeus and P. provincialis, the latter representing 95.64 % of the Tenebrionidae family. Of the Araneidae the most abundant species was Arctosa perita that represented 95-96% of the spider family. As the distance from the river increased species abundance increased as in the previous month and this was particularly true for T. saltator . For T. europaeus and the adults of P. provincialis the greatest number of individuals was caught at station 3. Abundance analysis carried out for the Coleoptera showed that in September only one species (P. provincialis ) was classified as abundant, whereas in May two species (P. provincialis , Hypocaccus dimidiatus ) presented values over 5 %. Influent species in September were only two (Trachyscelis apodioides and Gastroidea polygoni ) whereas in May four species occurred (Eugrapha arenaria , Trachyscelis apodioides , Halammobia pellucida and Aloconda sp). Also recedent species increased from 12 to 43 species when autumn captures were compared to spring ones. In September species richness had a total of 15 species that increased from 4 to 10 species proceeding from station 1 to station 5.  In May there was a general increase in species number with a total of 49 Coleoptera species present on the beach. In this case station 3 registered the highest number of species. The latter station also recorded the highest number of captured individuals in both seasons. Diversity indices showed differences according to the season, with higher values in spring. Furthermore α values increased as the distance from the river delta increased even if a significant linear trend was obtained only for the month of May. β diversity showed that the composition of the population changed at increasing distances from the river with a percentage of 50 % every 9m, 5m, 20m, 50m and 100m in September and every 100m, 8m, 12m, 20m and 33m in May. (Note, however, that the low value, 0.01, found at station 1 in May was biased by the low number of species present at that station). The evenness of the community presented the highest values at station 5  in both seasons, while more dominant species were found at station 3 and 1  in September and May respectively. Simple linear regression analysis showed clear trends proceeding from the station closest to the river delta to those farther away. Total captured individuals, T. saltator (adults and juveniles), T. europaeus (adults only in September), A. perita (only in May), the number of Coleoptera captured, species richness, α diversity indices (only in May) were significantly positively correlated with the distance from the river delta. Also sand parameters presented significant trends and these were positively (pH in September, beach width, MZ, and KG ) and negatively (pH in May, penetrability, beach slope, and σI) correlated with the distance from the river delta. When the total number of captured individuals was correlated with the environmental parameters positive correlations were obtained with sand moisture, penetrability, beach slope and width in September and with sand conductivity and beach width in May. In the latter month negative correlations were obtain only with penetrability and mean grain size. The species analysed separately gave different results accordingly. For T. europaeus similar trends were obtained for adults and juveniles with the difference that adults were correlated with more parameters than juveniles. A similar tendency occurred for T. saltator but in this case juveniles presented more significant correlations than adults. For the Coleoptera the number of individuals, and in particular P. provincialis , presented positive correlations with penetrability and beach width in September and negative ones with pH, beach slope, mean grain size in May. α diversity, calculated on species numbers and abundance, summarises these parameters. These were in fact correlated negatively with penetrability, beach width in September, and with pH, beach slope and mean grain size in May. Positive correlations were obtained with organic matter and penetrability in May and with mean grain size in September. β diversity showed significant correlations only with beach slopes and width in September and May respectively.

 

 

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