A Comparison of Two Laboratory Methods for Determining Sediment Organic Carbon Content in PEI Harbours January, 1999

Transcription

1 A Comparison of Two Laboratory Methods for Determining Sediment Organic Carbon Content in PEI Harbours January, 1999 Kendall R. Shaw PEI Department of Technology and Environment Charlottetown, PEI

2 Executive Summary Organic carbon content (OC) was measured in 1971 from samples taken at four PEI harbours using a CHN analyzer (Bartlett and Usher1971a and 1971b). Ten of these stations were re-sampled in 1997 (Shaw 1998) and OC was measured using the muffle furnace (MF) weight loss on ignition method. Nine of the ten 1997 results were higher than the 1971 results (Shaw 1998). In order to determine if this increase was due to an actual difference in OC over time or a difference in methodology, 38 of the 1971 stations were re-sampled and OC was measured using both laboratory methods. The MF produced significantly higher results than the CHN analyzer. There was no significant difference between the 1971 CHN results and the 1998 CHN results. The difference between the 1971 and 1997 OC measurements is likely due to a difference in methodology. A conversion factor of can be used to determine OC from MF organic matter content. Georgetown Harbour exhibited significantly higher CHN OC readings in 1998 than in It is unknown whether these elevated readings are consistent throughout Georgetown Harbour. i

3 TABLE OF CONTENTS EXECUTIVE SUMMARY i TABLE OF CONTENTS ii LIST OF FIGURES iii LIST OF TABLES iii INTRODUCTION 1 MATERIALS AND METHODS 2 Station Selection 2 Sample Collection 2 Core Analysis 2 Statistical Analysis 3 RESULTS AND DISCUSSION 3 CONCLUSION 10 LITERATURE CITED 11 APPENDIX A. ORGANIC CARBON CONTENT VALUES 12 APPENDIX B. MAPS OF SAMPLE LOCATIONS 14 LIST OF FIGURES ii

4 Figure 1: 1998 OC Values for Charlottetown Harbour 5 Figure 2: 1998 OC Values for Georgetown Harbour 5 Figure 3: 1998 OC Values for Summerside Harbour 6 Figure 4: CHN OC Values for Charlottetown Harbour 6 Figure 5: CHN OC Values for Georgetown Harbour 7 Figure 6: CHN OC Values for Summerside Harbour 7 Figure 7: MF OC Values for Stations Sampled in Table 1: LIST OF TABLES Method comparison for CHN and MF results determined by paired t test. 4 Table 2: Mean, maximum, minimum and SD values for organic 8 carbon content (%) Table 3. OC values (%) at 1997 stations for 1971 CHN and 1997 MF 10 determined with both conversion factors iii

5 Introduction Organic enrichment is one of the most common disturbances of marine macrobenthic communities (Weston 1990). Variation in the organic input to any area results in changes in chemical, physical, and biological factors which have effects on the benthic fauna present (Pearson and Rosenberg 1978). Consequences of the build-up of organic waste include an increase of oxygen consumption by organically enriched sediments, the formation of anoxic sediments and the production and release of harmful gases to the water column (Mattson and Linden 1983, Kaspar et al. 1985). Most of these conditions influence the abundance and composition of the benthic community (Schafer et al. 1995). It has also been indicated that high organic matter content in estuaries causes rapid depletion of the oxygen and permits the development of H2S emitting environments (Bartlett and Usher 1971a and 1971b; Bartlett 1973). In 1971 Bartlett (1973) determined organic carbon content (OC) in sediment cores taken from stations in Charlottetown, Summerside, Georgetown and Souris Harbours, Prince Edward Island. Organic carbon content was determined using a CHN analyzer. Ten sediment cores analyzed for the PEI Benthic Survey (Shaw 1998) were sampled at locations corresponding to Bartlett (1973). For these, organic carbon content was determined by weight loss on ignition using a muffle furnace (MF). The 1997 results were high relative to the 1971 results. This raises the question as to whether the difference is indicative of an organic carbon build-up over time or simply an artifact of the difference in methodology. In order to address this concern a selection of Bartlett s stations were re-sampled and OC was analyzed via both methods. This study design allowed for comparison between the two methodologies and between 1971 and 1998 results. 1

6 Materials and Methods Station Selection Thirty eight stations used by Bartlett (1973), which were representative of the full range of OC values present, were selected to be included in the study. Data from Souris Harbour was not available so this site was not included. The stations were mapped using MapInfo, and UTM coordinates were determined and entered into a hand held GPS unit for locating each station in the field. Due to the accuracy limitations of the original 1971 maps and the hand held GPS unit, the 1998 core samples may not have been taken at the exact 1971 locations. Maps of sample stations are included in Appendix B Sample Collection A 50 cm Wildco Core Sampler was used for sampling. Core liners were constructed from PVC tubing in 30 cm lengths. These liners had holes drilled every 2 cm, and were wrapped in duct tape to prevent water and sediment loss. Upon sample retrieval, the cores were capped with plastic caps, kept upright and transported to the lab. All sampling was conducted during the last two weeks of August Core Analysis All core samples were processed within 24 hours of collection but most were processed immediately upon return to the lab. Samples were refrigerated overnight if not processed immediately. Organic carbon content was determined in the surficial sediment layer of each core. Five milliliters of sediment were extracted from the surface layer of the core (i.e. the top hole that has underlying sediment) with a 5 ml cut-off syringe. The sediment was placed in a pre-weighed crucible and was placed in an Isotemp Incubator; Model 225D, at 60 o C for 48 hours. The crucibles were then weighed to determine sediment dry weight. Finally, they were placed in an Isotemp Muffle Furnace; Model 186A, at 600 o C 2

7 for one hour and weighed to determine organic matter content expressed as a percentage of sediment dry weight. This method for determining OM was done as per Hargrave (1995). OC is determined using equation 1 (Bartlett 1973). Organic Carbon (%) = Organic Matter (%)/1.7 [eq. 1] Approximately 20 ml of sediment was extracted from the surficial layer of the core and placed in a 30 ml plastic vial. These were delivered to the PEI Soil Lab in Charlottetown PEI to be analyzed for OC using a CHN analyzer. Statistical Analysis All statistical analysis was conducted using Systat 7. A paired t-test was used for all comparisons of OC values. To test for a difference between the two methodologies, the 1998 CHN results were compared to the 1998 MF results. This was done with all of the samples combined and for each site separately. To test for a difference in results over time, the 1971 CHN results were compared with the 1998 CHN results. Again this comparison was done with all samples combined and for each site separately. A p-value < 0.05 was considered to indicate a significant statistical difference. Results and Discussion The MF produced significantly higher OC results than the CHN analyzer for all sites combined (p = 0.000, Table 1). The MF produced higher OC results than the CHN analyzer at 29 of the 34 stations sampled (Figures 1, 2 and 3). This indicates a significant difference due to methodology. Since the muffle furnace method tends to produce higher OC results than the CHN analyzer, the increase in OC readings observed between 1971 (Bartlett 1973) and 1997 (Shaw 1998) is likely due, at least partially, to the difference in methodology. Table1. Method comparison for CHN and MF results determined by paired t test. 3

8 Site / All Sites Ch town G town S side Statistics Mean, 1998 CHN Mean, 1998 MF Mean Difference SD, Differences t p There was no significant difference between the 1971 CHN results and the 1998 CHN results for all sites combined (Figures 4, 5, and 6) (p value: 0.741). There was also no significant difference found between 1997 MF results and 1998 MF results (Figure 7) (p value: 0.252). This lack of significant difference in OC results over time, regardless of method, strengthens the argument that the increased levels in 1997 were due to different analysis methods. 4

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12 Table 2 presents summary statistics for the year-method categories used in the study. OC values are listed in Appendix A. Table 2. Mean, maximum, minimum, and SD values for organic carbon content (%) Method Year CHN 1971 MF 1997 MF 1998 CHN 1998 N Mean Maximum Minimum SD

13 Examining each harbour separately, the MF produced significantly higher OC results than the CHN analyzer in Charlottetown (Table 1, Figure 1) and Summerside (Table 1, Figure 3). There was no significant difference between methodologies for Georgetown (Table 1), but this is likely due to the small sample number as the muffle furnace produced higher results than CHN analyzer in 5 of the 7 stations (Figure 2). The 1971 the CHN OC results were significantly higher than the1998 CHN results for Charlottetown (Figure 4) but were significantly lower in Georgetown (Figure 5). In Summerside (Figure 6) there was no significant difference between the 1971 results and the 1998 results. This suggests that the degree of organic carbon build-up over time may be specific to each estuary, and location within the estuary. This supports Bartlett and Usher (1971a and 1971b) who state that the concentration of OC in an estuary varies from area to area and is related to the rate of primary production and the rate of deposition. The 1998 CHN OC readings at four of the Georgetown stations (33D, 33E, 35E, 35J) are substantially higher than 1971 CHN readings (Figure 5). There could be reason for concern if these increased readings actually represent an organic build-up over time. Further investigation could be done by re-sampling the stations and surrounding areas in Georgetown Harbour to determine the extent of these elevated readings and whether they are consistent spatially and temporally. A regression analysis determined that the relationship between 1998 MF organic matter and 1998 CHN OC is as follows: Organic Carbon Content (%) = Organic Matter Content (%) X [eq. 2] This conversion factor of is smaller than that suggested by Bartlett (0.588). Table 3 presents the 1971 CHN OC along with the corresponding 1997 MF OC values using both conversion factors. Using as the conversion factor adjusts the 1997 MF OC values so that they are similar to the 1971 CHN OC values (Table 3). 9

14 Table 3. OC values (%) at 1997 stations for 1971 CHN and 1997 MF determined with both conversion factors. Station 1971 CHN 1997 MF (0.588) 1997 MF (0.322) CH CH CH CH CH GT35E SS SS SS SS Conclusion Weight loss on ignition via the muffle furnace (MF) produced significantly higher organic carbon content values than the CHN analyzer. This significant difference could explain the difference between the results at 10 stations for 1971 and 1997 (Shaw, 1998), which were determined using different methods. The difference between the 1998 CHN OC values and the 1971 CHN OC values was dependent on the estuary. It should be noted that little can be deduced about the organic carbon build-up between 1971 and 1998 from the available data set of OC values. A sediment quality monitoring would be necessary to detect trends and changes over time. Georgetown Harbour could be resampled more extensively to determine if the elevated readings seen in 1998 are representative of an increase in sediment organic carbon and thus cause for concern. Literature Cited 10

15 Bartlett, G.A. and J.L. Usher. 1971a. Environmental analysis, ecology and nutrient study of Charlottetown Harbour and environs, Queen s University. Bartlett, G.A. and J.L. Usher. 1971b. Environmental analysis, ecology and nutrient study of Summerside Harbour and environs, Queen s University. Bartlett, G.A Environmental analysis and sediment investigation of Georgetown Harbour. Department of Regional Economic Expansion. Hargrave, B.T., G.A. Phillips, L.I. Doucette, M.J. White, T.G. Milligan, D.J. Wildish, and R.E. Cranston Biogeochemical observations to assess benthic impacts of organic enrichment from marine aquaculture in the Western Isles region of the Bay of Fundy, Can. Tech. Rep. Fish. Aquat. Sci Kaspar, H.F., P.A. Gillespie, I.C. Boyer, and A.L. MacKenzie Effects of mussel aquaculture on the nitrogen cycle and benthic communities in Kenepuru Sound, Marlborough Sounds, New Zealand. Mar. Biol. 85: Mattson, J. and O. Linden Benthic macrofauna succession under mussels, Mytilus edulis L. (Bivalvia) cultured on hanging long lines. Sarsia. 68: Pearson, T. and R. Rosenberg Macrobenthic succession in relation to organic enrichment and pollution of the marine environment. Oceanogr. Mar. Biol. Ann. Rev. 16: Ritz, D.A., M.E. Lewis, and M.A Shen Response to organic enrichment of infaunal macrobenthic communities under salmonid seacages. Mar. Biol. 103: Schafer, C.T., G.V. Winters, D.B. Scott, P. Pocklington, F.E. Cole, and C. Honig Survey of living foraminifera and polychaete populations at some Canadian aquaculture sites: potential for impact mapping and monitoring. J. Foram. Res. 25: Shaw, K.R PEI Benthic Survey. Prince Edward Island Technical Report of Environmental Science. 94 pp. Weston, D.P Quantitative examination of macrobenthic community changes along an organic enrichment gradient. Mar. Ecol. Prog. Ser. 61:

16 APPENDIX A. ORGANIC CARBON CONTENT VALUES SITE STATION Easting Northing OC_71_CHN OC_97_MF OC_98_MF OC_98_CHN 12

17 (%) (%) (%) (%) Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Charlottetown CH Georgetown GT35E Georgetown GT35G Georgetown GT35J Georgetown GT35K Georgetown GT35L Georgetown GT35M Georgetown GT33E Georgetown GT33D Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS Summerside SS

18 APPENDIX B. MAPS OF SAMPLE LOCATIONS 14

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