Of Mayflies in Riffles and Pools - Case Study Example

www.academia-research.com Sumanta Sanyal Incidence of Mayflies d: Comparative Study: Incidence of Mayflies in Riffles and Pools Individual Study Abstract The aim of this study was to compare the relative incidence of mayfly larvae, nymphs, in pools and riffles. The principal hypothesis was that the number observed in riffles would be much more than that in pools. Samples from 10 sets of pools and riffles were taken in the summer of 2004 in Wales, the UK. The results bore out the hypothesis showing that the number of nymphs in riffles was considerably more than that observed in pools. This was true in all cases without exception. This is believed to be because riffles are fast-flowing aquatic environments with high levels of dissolved oxygen while pools are relatively slow-flowing environments where the dissolved oxygen levels are not as high as in riffles. The nymphs prefer high dissolved oxygen level environments. Hypothesis The number of mayfly larvae, nymphs, will be greater in riffles than in pools. Null Hypothesis There will be no difference in the number of nymphs taken from riffles and pools. Introduction For the purpose of this study it is necessary to introduce the mayflies (Ephemeroptera) because they are such a primitive order of insects that there is much that is unique about their morphology. This uniqueness has to be highlighted to enable better understanding of the experiments conducted to assist this study and the conclusions derived therefrom. Classification Using the Linnaean system of hierarchical classification mayflies can be classified as: Superphylum – Arthropoda Phylum – Entoma Subphylum – Uniramia Superclass – Hexapoda Class – Insecta Subclass – Ptilota Infraclass – Palaeopterygota Order – Ephemeroptera (Williams & Feltmate, 1992. Derived from “Soil & Water Conservation Society of Metro Halifax” Website, 2004) Simply put mayflies belong to Class Insecta Order Ephemeroptera. They belong to the infraclass paleoptera and have primitive wings that cannot be folded over their backs. Fossil records reveal that they may have evolved during the carboniferous period 280-360 mya and their evolutionary history is closely associated with development of wings in Class Insecta as a whole. Modern day mayflies number about 4000 species distributed among 20 families and most are associated with running water (Brooks, Steve, A Natural History of Dragonflies, Mayflies and Stoneflies). Order Ephemeroptera is well-distributed across the globe except for the two polar regions – the Arctic and the Antarctic – and oceanic islands though it is well-represented in New Zealand (Order Ephemeroptera, “Soil & Water Conservation Society of Metro Halifax” Website, 2004). Life History The name of the order Ephemeroptera is essentially derived from the fact that adult phase mayflies survive for a very ephemeral period of 2 hr to 3 days. They are unique insects in that they have two adult phases both of which are winged and ephemeral to the tune of 1-2 hr to maximum 14 days. At adult stages the insects do not feed and expend all their time on mating. The nymphs are ubiquitous and are usually found in shallow streams and littoral areas of lakes. Nevertheless, many species are restricted to specific substrata of macrophytes, sediments of waveswept or moving stream areas and specifically sized particle sediments (Order Ephemeroptera, “Soil & Water Conservation Society of Metro Halifax” Website, 2004). Mayfly larvae, or nymphs, are diverse but they can be broadly categorised into four groups: burrowing, flattened, swimming and creeping. Special adaptations enable them to adjust to these functions, all of which are linked to acquisition of oxygen while feeding. Burrowing species bury themselves in firm silty sand so that they can feed as well as breathe through their gills without getting washed away by the current. Nymphs with flattened bodies are streamlined so that strong currents do not wash them away as they stick to surfaces of rocks and other underwater strata as they feed (Habitat/Feeding Habits & Adaptations, Mayfly Central). Nymphs of almost all species have three tails, a constant feature that enables distinction from aquatic larvae of other insect orders like stoneflies. Habitat Adult flies are naturally not aquatic but the nymphs are and they prefer both stagnant and flowing water habitats. The habitat ranges that pertain to this study are two and they are: lotic erosional (running water riffles), and lotic depositional (running water pools and margins). Diet Mayflies are principally detrivores and grazers and collector-gatherers and their diet is primarily composed of algae, diatoms and organic detritus though some species like Oligoneuriidae are filter-feeders while others are predatory (Williams & Feltmate, 1992. Derived from “Soil & Water Conservation Society of Metro Halifax” Website, 2004). Incidence in the UK Since sampling of insects for this comparative study has been done in Wales, the UK, it is germane to observe that the Ephemeroptera Recording Scheme, UK, November 2000, lists 51 species of mayflies in the British Isles. From past evidence the nymphs of these incumbent species prefer a varied aquatic environment comprised of both stagnant and fast and medium flowing water. Biological Knowledge to Support the Hypothesis From the biological facts available on mayflies it is deduced that nymphs are specially adapted to an aquatic life with varying morphological features that can enable them to exist in differing environments with ease. Nymphs of many species have flattened stream-lined bodies that enable them to cling onto substrata while feeding without being washed away by the current. These types also depend on a large part on the fast-flowing water washing over their gills thus assuring a constant supply of oxygen. It is also significant that nymphs attuned to benthic environments have larger gills to produce larger areas for oxygen collection (Habitat/Feeding Habits & Adaptations, Mayfly Central). It is thus significant that the various adaptations of the nymphs of various species are mostly targeted towards enabling a high level of steady oxygen supply. Methodology The experiments were conducted in Wales, in the United Kingdom. The experimental habitats were chosen as riffles and pools, both lotic environments with varying degrees of fast-flowing water. 10 experiments were conducted in the summer, 2004, when the water temperatures were moderate and incidence of mayfly nymphs numerous. For all 10 experiments samples were taken from both a riffle and a pool. Separate pools and riffles were chosen for each experiment. At the onset of the experiments the nitrite levels of the water in the experimental pools and riffles were checked to assure of purity. Mayfly nymphs are extremely sensitive to pollutants and pollution and a high significance in their incidence patterns. For each experiment the nitrite levels of the water in the pool and riffle were assessed and recorded. For each experiment the temperatures of each pool and riffle were checked with a temperature probe and recorded. Similarly, for each experiment the dissolved oxygen content in the water of the pool and riffle was checked and recorded. Flow rates of the pool and riffle for each experiment was also checked and recorded. The rate was assessed along a straight stretch of pool or riffle. A meter rule and orange dye, which is a conspicuous non-toxic variety that floats just under the surface of the water, was chosen. The dye was allowed to run the entire length of the rule and timed to get speed of flow. For each reading three runs were made and the average taken. A kick-net was used to make the sample catch. For each sampling it was kicked for exactly 30 s. After the sample nymphs were taken out it was rinsed to make sure that everything extracted out of the water was accounted for. This was done every time it was used. For each experiment pool and riffle the caught mayfly nymphs were put into a separate paint palette using a pipette. The pools were sampled first after which the riffles were. The nymphs were ascertained as being larvae of mayflies by ensuring that each had three tails, a confirmed sign for mayfly nymphs, to distinguish them from nymphs and larvae of other orders like stoneflies. Ethical Considerations Some essential ethical considerations were made during the conduction of the experiments. After being extracted from the habitats the sample nymphs were constantly kept in clean and clear water to ensure their survival. The test kits were not thrown away to preserve the pristineness of the experimental environment. The level of disturbance to the experimental environment was kept to a minimum, well within acceptable limits. All the extracted sample nymphs were put back into their habitats after the conclusion of the experiments. Results 8 types of nymphs, possibly signifying 8 different species, were found in the samples from all experiments. The nitrite concentrations for all the sampled pools and riffles were below 0.3 %. The findings from all the 10 experiments have been tabulated in Tables 1-10 (Appendices 2-11). Experimental temperatures ranged between 17-C with the lowest at C, for the pool in experiment 1, and the highest at C, for the riffle in experiment 5. Temperatures for the pools were always lower than that of riffles in most experiments though in experiments 3 & 4 they were exactly the same. Pool depths were always less than that of riffles in all the experiments. This shows that the sedimentation rates for the pools is higher than that of the riffles, which are erosional in nature. For the pools the depths ranged from 0.09 m, pool in experiment 5, and 0.17 m in experiments 6 & 7. The minimum riffle depth recorded was 0.24 m for experiment 7 and the maximum 0.36 m for experiment 2. It was found that the riffle flow rates were higher than that of the pools in all the experiments. This is quite in line with the expectation that pools are slower-moving habitats than riffles. Oxygen concentration recorded for the pools were lower in all experiments than those of riffles. A scatter diagram (Appendix 1) has been plotted on a graph with x-axis as the number of nymphs and y-axis as oxygen concentration for both riffles and pools. A set pattern emerges for incidence of nymphs in both riffles and pools and the patterns are interpreted in the discussions. Discussion Nitrite levels below 0.3 percent suggest low levels of organic pollution. This is especially significant as such pollution reduces dissolved oxygen levels by promoting algal and other microbial growth. This bodes well for mayfly incidence as the insects are sensitive to such pollution and increase in their numbers is good for this biome as they are a major link in the food chain serving as food for fish and other small aquatic creatures. The mayfly is an important biological indicator to water quality, especially in relation to temperature and dissolved oxygen content. The larvae live only in the cleanest water (Mayflies (Ephemeroptera) ). Thus, incidence by healthy numbers of these insects in the experimental area bodes well for the environment there indicating that high levels of environmental purity is being sustained. It is accepted fact that higher temperatures, up to certain levels for water specifically, facilitate dissolution of oxygen in water. In all the experiments the temperatures recorded for both the pools and the riffles lay within a small range of C. This necessarily preempts any possibility of increased oxygen content in either pools or riffles resulting from higher temperatures. This also preempts possibilities of variation in mayfly larvae number due to temperature variations as they are somewhat sensitive to such variations, especially seasonal ones. The depth and flow rate measurements for riffles were all higher than those of the pools. This suggests that the higher figures for the riffles do have positive co-relations with the uniform higher oxygen concentrations of the riffles. This is quite in line with expectations generated by hydraulics which suggest that the greater flow rates coupled with the greater depths enhanced the oxygen concentrations of the riffles compared to the pools. The scatter diagram (Appendix 1) demonstrates that a clear pattern is visible for both riffles and pools for correlation between incident number of nymphs and oxygen concentration. For the pools the maximum concentration of points (6: , ,, , and ) are in the region of oxygen concentration 13-15 with number of nymphs ranging from 12-15. The other points , and appear to be in region of higher oxygen concentration 15-17 but have less number pf nymphs 10-12. is off-track and so is discounted. These two concentrations of points suggest that oxygen concentration is optimum at the range 13-15 where most nymphs are evident. Any rise in concentration causes the number of nymphs to fall off. For the riffles the maximum concentration of points (6: , , , and ) are in the region of oxygen concentration range 22-25 with number of nymphs ranging from 54-60. All the other points are scattered away from this central region with either lower or higher concentration of oxygen and lower number of nymphs suggesting that the optimum oxygen concentration range is 22-25. Except for which is within this oxygen concentration range but with considerably less number of nymphs at 48 (it is discounted) the other 4 points lie either lower or higher of this range and have less number of nymphs. From these two observations from the scatter diagram it is evident that the optimum oxygen concentration ranges for pools and riffles are separate suggesting incidence of two separate species, or groups of species, of nymphs whose dissolved oxygen tolerances are different. The species’ inhabiting the pools have a higher tolerance to oxygen deficiency than the ones that inhabit the riffles. The null hypothesis is easily rejected with the Mann-Whitney test. Taking all 10 observations of nymph numbers in pools as Sample 1 and all 10 observations of nymph numbers in riffles as Sample 2 it is evident that all 10 observations of Sample 2 are greater than all 10 of Sample 1. Thus, U = 10 demonstrating that there is absolutely no possibility of chance that nymph numbers in riffles are greater than those in pools (Mann-Whitney U, Wikipedia Online Encyclopedia, Last modified September 2005) Anomalies Another significant anomalous occurrence revealed by the findings from the experiments is that 8 types of nymphs, possibly signifying 8 different species of mayflies, were found among all the samples. This is of some importance in the sense that it is known that certain types of mayfly nymphs have special adaptations that allow them to survive in slow-moving water. Such adaptations may be in the form of larger gills which ensure larger surface area available for gaseous exchange allowing the larvae to subsist in water with relatively low oxygen content (Brooks, Steve, A Natural History of Dragonflies, Mayflies and Stoneflies). Thus, it is essential to accurately ascertain the species of the catches so that special adaptations that can surmount relative oxygen deficiencies of the water and create anomalies within the interpretations can be successfully pre-empted. It is especially necessary to ascertain that all species prefer habitats with high flow rates so that anomalies resulting from intermixed species preferring high and low flow rates do not result. If there is intermixture of species, as is suggested by the scatter diagram which shows different optimum oxygen concentration tolerance levels for the nymphs from riffles and pools, then it is necessary to ascertain exactly what manner of adaptations the two sets of nymphs adopt to thrive in riffle and pool habitats. Another saliently anomalous point would be that if different species inhabit the different habitats – riffles and pools – why is the number of nymphs in the riffles so much more than those in pools? This may be because of a possible factor: the number of intermixed species in the riffles are more than those in the pools so that numbers are habitually higher in the riffles, the more beneficial habitat, than in the pools, the more hostile one, which may be inhabited by lesser number of species thus having lesser number of nymphs. Conclusion The hypothesis that there will be more nymphs in riffles than in pools is borne out in a certain manner by the findings of the study. It is found that greater availability of oxygen at an optimum concentration increases incidences of mayfly larvae but the optimum concentration range possibly varies among species’ inhabiting different habitats – pools and riffles. Also, since the number of nymphs is considerably higher in all riffle samples compared to the pool ones it can be inferred that the riffle environment may be a more beneficial one for mayflies than the pool one though different species may inhabit both. It can also be inferred that species’ inhabiting riffles are more prolific than ones inhabiting pools because of the more beneficial environment. References Brooks, Steve, A Natural History of Dragonflies, Mayflies and Stoneflies, Natural History Museum Website, 2005. Extracted on 13th November, 2005, from: http://www.nhm.ac.uk/nature-online/life/insects-spiders/fathom-dragonflies/a-natural-history-of-dragonflies-mayflies-and-stoneflies.html Ephemeroptera Habitat Classification System. Extracted on 13th November, 2005, from: http://www.science.mcmaster.ca/Biology/Harbour/SPECIES/MAYFLIES/AQHAB.HTM Ephemeroptera Recording Scheme, The British Species Section. Extracted on 13th November, 2005, from: http://www.ephemeroptera.pwp.blueyonder.co.uk/ Habitat/Feeding Habits & Adaptations. Extracted on 13th November, 2005, from: http://www.science.mcmaster.ca/Biology/Harbour/SPECIES/MAYFLIES/HABITAT.HTM Mann-Whitney U, Wikipedia Free Online Encyclopedia. Last modified September 10, 2005. Extracted on 13th November, 2005, from: http://en.wikipedia.org/w/index.php?title=Mann-Whitney_U&printable=yes Mayflies (Ephemeroptera). Extracted on 13th November, 2005, from: http://people.westminstercollege.edu/faculty/tharrison/emigration/mayfly.htm#Top Order Ephemeroptera (Mayfly), Soil & Water Conservation Society of Metro Halifax, July 5, 2004. Extracted on 13th November, 2005, from: http://lakes.chebucto.org/ZOOBENTH/BENTHOS/iii.html#linnean Appendix 1. Scatter Diagram Appendices 2-11 comprise the 10 table which I am not appending to conserve file length. Read More