Eutrophication and Changes in Algal Growth on Coral Reefs. Jen Massey and Cassandra Reyes-Jones

Transcription

1 Eutrophication and Changes in Algal Growth on Coral Reefs Jen Massey and Cassandra Reyes-Jones Abstract Freshwater runoff naturally delivers large amounts of nutrients to the near shore environment resulting in macroalgae proliferation. Changes in land use (e.g. agriculture, urbanization, etc.) have dramatically increased nutrient levels with a corresponding change in the abundance of macroalgae. Using the Viapahu River (Moorea, French Polynesia) as our study system, we tested the hypothesis that riverine input affects macroalgal abundance and composition. Nutrient levels in our sample site changed significantly with riverine enhancement. There was a significant difference in the levels of total nutrients before and after the rainy season began, although the overall abundance and spatial location of algae did not change significantly. The site composition significantly changed with riverine input and there was a significant difference in the overall substrate composition of the site after the rains began. Algal growth plates were set out to monitor growth. Picture analysis of the plates showed that with increasing distance from the river mouth, abundance of algal growth did not change dramatically. However, the type of algae on the plates changed with distance. We found that some algal spatial patterns correlated with nutrient and substrate patterns. Introduction Distribution and abundance of benthic organisms are influenced by five sets of factors: () resource availability (light, nutrients, substrate), () supplyside factors (fecundity, dispersal, settlement, recruitment), (3) physical stress gradients and disturbance establishment (depth, wave exposure and cyclones, temperature, freshwater, sediment deposition), (4) species interactions (competition and predation, mutualisms), and (5) the historical effects of and interactions among these factors (Lewin, 986). For coral reef macroalgae, research has focused on two factors: herbivory and water quality, particularly changes in nitrogen and phosphorus. Freshwater runoff naturally delivers large amounts of nutrients to the near shore environment. However, changes in land use (e.g. agriculture, urbanization, etc) have dramatically increased nutrient levels with a corresponding change in the abundance of macroalgae. Macroalgae play important roles in the coral reef community and are critical to its ecological, environmental, aesthetic and socio-economic value. Macroalgae abundance can also be an indication of coral reef health. Increases in algal growth have been interpreted as evidence for reef degradation caused by increase in sediment and nutrients or by a reduction in the numbers of herbivorous fishes due to overfishing (McCook, 999) or to a loss of invertebrate herbivores such as sea urchins (Hughes, 994). Another factor that is linked to human induced increases in macroalgae abundance is high sedimentation and increased turbidity resulting from increased freshwater runoff. Light is necessary for

2 photosynthesis to take place in symbiotic unicellular zooxanthallae to which most, if not all, of the corals nutrients reside (Rogers, 99). Hence increases in runoff can result in reef degradation via two processes: decreased light via turbidity and increased cover of macroalgae due to increases in nutrients. Degradation of coral reefs has brought widespread scientific, management, and public concern since it appears to be the result of an increase in anthropogenic influence (McCook, 999). Using the Viapahu River as our study system, we tested the idea that riverine input affects macroalgal abundance and composition. We predicted the following: ) The level of nutrients will change with the onset of the rainy season because of increased flow from the river. ) The spatial pattern of nutrient levels will be consistent with riverine enhancement. 3) The spatial pattern of algal abundance and composition will be consistent with the patterns for nutrients, thereby consistent with riverine input. Materials and Methods Study Area Our survey site was located in Moorea, French Polynesia at the mouth of the Viapahu River on the northern tip of the peninsula bounded by Opunahu Bay and Cook s Bay. The Viapahu River is located on the east side of the Sheraton Hotel and extends 3 meters up Mount Rotui. The sample area was 48 square meters in the lagoon directly in front of the river mouth. The maximum depth was.4 meters at 64 meters away from the river mouth. The site composition was mostly sand, rubble, dead coral, and sporadic live coral heads. We chose this site for the small size of its outfall, altitude of the source, and logistic accessibility. A small freshwater discharge has a localized influence and is a more tractable system than a large one. However, in our case, the presence of a nearby hotel and its associated potential outflow may complicate the predicted patterns. Patterns of Algal Composition To determine the composition of algae near the mouth of the river and the surrounding lagoon area we did the following survey (Fig. ): Salt water Shore Fresh H Imput Figure - Transects (Figures and are a possible pattern of the freshwater input flow and a schematic of our sampling methods. The prediction is based on a fluid dispersal model.) On November 7, 4, before the rainy season began, and on December 3, 4, during the rainy season, we ran nine transects from shore using geometric spacing from the river mouth. The transects were at and at, 4, 8, 6 meters away from the river mouth on both the east and west sides of the river mouth (Fig. ). Each transect was 64 meters long. Using ½ m x ½ m PVC quadrats, samples were done on each transect along a geometric progression from shore. Each transect was sampled with a quadrat at, 4, 8, 6, 3, and 64 meters. The percent coverage of algae species and substrate

3 matter was recorded from each quadrat. We used a geometric sampling scheme (centered at the river mouth) because we expected the signal of nutrient enhancement to be fairly local. The concentration of sampling effort close to the river mouth ensured that we would be able to detect a riverine signal if one existed. The algae we sampled for were: Enteromorpha flexuosa Ulva lactuca Boodlea kaeneana Ventricaria ventricosa Halimeda sp. Neomeris vanbosseae Green turf algae Dictyota sp. Padina boryana Colpomenia sinuosa Sargassum mangarevense Turbinaria ornata Gelidiella sp. Actinotrichia fragilis Galaxaura filamentosa Peysonnelia sp. Amphiroa sp. Hydrolithon sp. Hypnea sp. Red turf algae Unidentified cyanobacteria Lyngbya majuscula Phormidium sp. Pattern of nutrients To test if the river was a high source of nutrient input to the nearby reef, we based our sampling effort on the results of the initial survey described above. In part, this allowed us to use the information on algal composition to predict the spatial pattern of nutrients. Accordingly, we took water samples of the river and at various points within the lagoon (Fig. ). Figure - Nutrient and temperature date sites We tested the water samples using CHEMets Kits for levels of nitrates and phosphates. We tested the water before the rainy season on November 8, 4 and after the rainy season began on December, 4. The water was tested on the same days for temperature at 9 points in the lagoon. Patterns of algal growth To test if algal growth varied as a function of proximity to the river discharge, the following experiment was performed. We constructed algal growth plates using cm x cm clay plates fastened to cinder blocks and covered in wire mesh cages to prevent herbivory (Fig. 3). There were a total of twenty plates, four plates per cinderblock on a total of five cinderblocks. The blocks were all initially placed in the same area in the lagoon on November, 4 to obtain an initial layer of algal growth. After an algal film developed, the blocks were relocated on November 7, 4 to 8m, 4m, 3m, 48m, and 64m on the m transect. On November 9, 4, digital photos were taken of each plate to define the initial growth state. Algae were allowed to grow on the plates with protection against herbivory using wire mesh cages (Fig. 3). On December 4, 4, digital photos were taken of each of the plates to define the final growth states.

4 composition of rubble (p=.43) and dead coral (p=.). Figure 3- Algal growth plates set-up on cinderblocks. Data analyses Data were analyzed using the software package SYSTAT. Results Nutrients The nutrient levels in our sample site changed significantly with riverine enhancement. There was a significant difference (p=.) in the levels of total nutrients before and after the rainy season began. Increased levels of nitrate (NO 3 - ) showed a significant difference (p=.). Phosphate (PO 4 - ) also showed a significant difference (p=.6). Algal Growth Plates Photo observations allowed us to monitor the differences in algal growth with respect to distance from the river mouth. Analysis of the photos showed that with increasing distance from the river mouth, abundance of algal growth did not change dramatically. However, the type of algae on the plates changed with distance. Discussion Nutrients We only had enough supplies to make 3 water nutrient measurements. Since we needed to test pre- and postrain season, only two sampling dates could occur. Ideally, we would have tested the water at each of our quadrat sites. Although only two samplings occurred, we did determine a nutrient pattern (Fig. 4) Algal abundance and patterns The overall abundance and spatial location of alga before and after the rains did not change significantly (p=.75). However, some of the algal spatial patterns did follow patterns of nutrient changes and riverine input a) Phosphate (PO 4 - ) 7 6 PO PO Site Composition The site composition significantly changed with riverine input. There was a significant difference (p=.) in the overall composition of the site after the rains began. The differences were mainly lead by changes in sand composition (p=.5) and live coral composition (p=.8). No significant difference showed in the b) Nitrate (NO 3 - ) - NO Figure 4- Nutrient pattern before () and after () rain. Trace phosphates were present before rain began (a ) and showed a strong increase as rain continued (a ). No nitrates were present before rain (b ) and also showed a strong increase as rain continued (b ). - - NO

5 These patterns support our first and second hypotheses that the nutrient levels increased with increased riverine discharge, and that the spatial pattern of nutrients were correlated with riverine enhancement. Algal Abundance and Patterns The overall abundance and spatial location of alga before and after the rains did not change significantly. However, some of the algal spatial patterns did follow patterns of nutrient and substrate changes from riverine input. Initially, a green turf alga was present on the rubble in front of the river mouth (Fig. 5a ). After the rain began, the increase in riverine discharge pushed back the rubble that was initially present in front of the river mouth. The rubble was replaced by sand which shows a correlation with the spatial pattern seen for green turf alga (Fig. 5a ). The increase in flow from the riverine appears to have pushed the spatial pattern of Hydroclathrus clathratus back when compared to the initial state before the rain began (Fig. 5b). The spatial pattern of the unidentified cyanobacteria appeared to have a correlation with the nutrient spatial pattern. The change in substrate composition did not seem to have a major effect on the spatial pattern of the unidentified cyanobacteria (Fig. 5c) a) Green turf alga - GREENTURFA GREENTURFA HYDROCLATHRU b) Hydroclathrus clathratus UNIDENTIFIED c) Unidentified cyanobacteria Figure 5- Spatial patterns of algal growth before () and after () rain began. Site Composition The site composition significantly changed with riverine input. Flow rates of Viapahu River were taken inside the river when the rainy season began (Fig. 6). The flow steadily increased, changing the bottom composition of the surrounding lagoon. Initially, rubble was most dense around the river mouth (Fig. 7a ). When the rains began and continued and river flow increased, the rubble was pushed back and replaced by sand (Fig. 7a ). As distance from the river mouth increased, a gradient of decreasing rubble to increasing sand occurred. Along the same gradient, further distance from the river mouth, the bottom composition shifted from only sand to sand with patchy coral, dead and/or live. Errors in data collection due to our transects and quadrats not being permanent, caused the generated substrate maps to show a shift in the location of live and dead coral (Fig. 7cd). Visibly analyzing the lagoon, there was no obvious shift in live or dead coral locations and any mapped differences were due to sampling error. No pattern could be found for live and dead coral HYDROCLATHRU UNIDENTIFIED

6 FLOW (L/sec) DAY Figure 6- Increase in flow of Viapahu River due to rainfall a) Rubble b) Sand c) Dead Coral d) Live Coral RUBBLE SAND DEADCORAL LIVECORAL Figure 7- Substrate Composition Maps ) Before Rain. ) After Rain RUBBLE SAND DEADCORAL LIVECORAL Changes in site composition could lead to possible changes in spatial location of algae. If time had permitted these spatial changes could have been observed. Further data collection could have related changes in site and algal composition to possible correlations with increased riverine input. Algal Growth Plates Time was a limiting factor in our experiment. Initially, we wanted to set the plates out and be able to identify the genera that grew on the plates and get percent cover data to determine changes in growth on a spatial scale. Unfortunately, we could only allow 8 days of growth to occur. Photo observations of the growth did allow us to note a difference, but identification and percent cover data could not be received from the photos. The type of growth was also dependent on substrate. Growth on the actual cinderblock itself was different than growth on the plates on that same block. The appearance of different algal species on the plates showed a correlation with distance from the river mouth. Although the species were not identified, it can be assumed that proximity to the river mouth and higher nutrient levels affected growth of particular algal species. Temperature and Current Temperature and current data were tested as possible abiotic influences on algal growth spatial patterns. Since there was not a significant change in algal growth patterns over the course of this experiment, abiotic factors could not be linked to having an effect. Temperature data was collected along shore using YSI 85 Oxygen Conductivity Salinity & Temperature probe on the two days we sampled for nutrients. Before the rain began, the

7 temperature had a steady increase along shore. After the rains began, the riverine discharge influenced the decrease in temperature. Current speed and angle also changed after the rain began (Fig. 8 and 9). TEMPERATURE TEMPERATURE Figure 8- Temperature data for before () and after () rain began Current (m/sec) ().. ()..3.4 Figure 9- Polar graph of current speed and angle before () and after () rain. Conclusion Current Angle Although our experimental time was limited, we found patterns to support our hypotheses. To gain an overall knowledge about the extent to which freshwater runoff effects near - shore ecosystems, a similar survey would need to be conducted over a larger temporal scale. The specific algal spatial pattern would have to be determined and monitored over multiple rainy seasons to determine how the freshwater runoff has an effect on changing that spatial pattern. Our survey was only one small spike on the overall map that would need to be established to conclude that the riverine input has a direct effect on algal growth, and indirectly on coral health. Acknowledgments We would like to thank our amazing instructors, Pete Raimondi for his thorough support and assistance in every step of this project, and Giacomo Bernardi for his support, sarcasm, and his undying interest in algae and nutrients. Thanks to our awesome TA s Jared Figurski, Rikki Preisler, and especially Dawn Jech for her photographic talents and for being a mutual algae geek. Thanks to Donnie from Bali Hai, Pete DalFerro, and Chris Reeves for the pursuit of the only clay plates on the island. Thanks to Tanetoa for the refreshments that kept us going strong on long, hard working days. Thanks to Steve Clabuesch, Lauren Scheinberg, Patrick Berk, Liz Atwood, Moorea Anderson, and Robyn Cole for supporting us in the field and in the pool down at the Sheraton. Much love and thanks to the Bio 6 class for making Super Nerd Science Camp the best ever.

8 References Hughes, TP Catastrophes, phase shifts, and large-scale degradation of a Caribbean coral reef. Science 65: Lewin, R Supply-side ecology. Science 3: 5-7. McCook, LJ Macroalgae, nutrients and phase shifts on coral reefs: scientific issues and management consequences for the Great Barrier Reef. Coral Reefs 8: Rogers, C. 99. Responses of coral reefs and reef organisms to sedimentation. Mar. Ecol. Prog. Ser. 6: 85-