Richard A. MacKenzie

Impacts of climate change on mangrove ecosystems: a region by region overview

Abstract Inter-related and spatially variable climate change factors including sea level rise, increased storminess, altered precipitation regime and increasing temperature are impacting mangroves at regional scales. This review highlights extreme regional variation in climate change threats and impacts, and how these factors impact the structure of mangrove communities, their biodiversity and geomorphological setting. All these factors interplay to determine spatially variable resiliency to climate change impacts, and because mangroves are varied in type and geographical location, these systems are good models for understanding such interactions at different scales. Sea level rise is likely to influence mangroves in all regions although local impacts are likely to be more varied. Changes in the frequency and intensity of storminess are likely to have a greater impact on N and Central America, Asia, Australia, and East Africa than West Africa and S. America. This review also highlights the numerous geographical knowledge gaps of climate change impacts, with some regions particularly understudied (e.g., Africa and the Middle East). While there has been a recent drive to address these knowledge gaps especially in South America and Asia, further research is required to allow researchers to tease apart the processes that influence both vulnerability and resilience to climate change. A more globally representative view of mangroves would allow us to better understand the importance of mangrove type and landscape setting in determining system resiliency to future climate change.

A comparison of the vegetation and soils of natural, restored, and created coastal lowland wetlands in Hawai'i.

The loss of coastal wetlands throughout the Hawaiian Islands has increased the numbers of created (CW) and restored (RW) wetlands. An assessment of these wetlands has yet to occur, and it has not been determined whether CWs and RWs provide the same functions as natural wetlands (NWs). To address these concerns, vegetation and soil characteristics of 35 wetlands were compared within sites along hydrologic gradients and among sites with different surface water salinity and status (i.e., CW, RW, NW). Only 16 of 85 plant species identified were native and three of the four most abundant species were exotic. Vegetative characteristics differed primarily across salinity classes, then along hydrologic zones, and to a lesser extent among CWs, RWs, and NWs. Soil properties exhibited fewer differences across salinity classes and along hydrologic zones and greater differences among CWs, RWs, and NWs. The dominant presence of invasive species in coastal Hawaiian wetlands suggests that it will be difficult to locate reference sites that can be used as restoration targets. Differences in edaphic characteristics suggested that RWs/CWs do not exhibit the same functions as NWs. Future restoration and creation should include planting of native vegetation, controlling invasive vegetation, and alleviating inadequate soil conditions.

Climate change and land use drivers of fecal bacteria in tropical Hawaiian rivers

Potential shifts in rainfall driven by climate change are anticipated to affect watershed processes (e.g., soil moisture, runoff, stream flow), yet few model systems exist in the tropics to test hypotheses about how these processes may respond to these shifts. We used a sequence of nine watersheds on Hawaii Island spanning 3000 mm (7500-4500 mm) of mean annual rainfall (MAR) to investigate the effects of short-term (24-h) and long-term (MAR) rainfall on three fecal indicator bacteria (FIB) (enterococci, total coliforms, and ). All sample sites were in native Ohia dominated forest above 600 m in elevation. Additional samples were collected just above sea level where the predominant land cover is pasture and agriculture, permitting the additional study of interactions between land use across the MAR gradient. We found that declines in MAR significantly amplified concentrations of all three FIB and that FIB yield increased more rapidly with 24-h rainfall in low-MAR watersheds than in high-MAR watersheds. Because storm frequency decreases with declining MAR, the rate of change in water potential affects microbial growth, whereas increased rainfall intensity dislodges more soil and bacteria as runoff compared with water-logged soils of high-MAR watersheds. As expected, declines in % forest cover and increased urbanization increased FIB. Taken together, shifts in rainfall may alter bacterial inputs to tropical streams, with land use change also affecting water quality in streams and near-shore environments.

Nekton Communities in Hawaiian Coastal Wetlands: The Distribution and Abundance of Introduced Fish Species

Nekton communities were sampled from 38 Hawaiian coastal wetlands from 2007 to 2009 using lift nets, seines, and throw nets in an attempt to increase our understanding of the nekton assemblages that utilize these poorly studied ecosystems. Nekton were dominated by exotic species, primarily poeciliids (Gambusia affinis, Poecilia spp.) and tilapia. These fish were present in 50–85% of wetlands sampled; densities were up to 15 times greater than native species. High densities of exotic fish were generally found in isolated wetlands with no connection to the ocean, were often the only nekton present, were positively correlated with surface water total dissolved nitrogen, and were negatively correlated with native species richness. Native species were present in wetlands with complete or partial connection to the ocean. Additional studies are needed to document exotic fish impacts on native fish and bird habitat and whether native fish communities can contribute to invasion resistance of coastal wetlands. Future wetland restoration should include exotic fish eradication, maintenance of hydrological connection to the ocean, or programs to prevent future introductions in order to create wetlands that support native-dominated nekton communities.

Nitrogen source tracking with δ15N content of coastal wetland plants in Hawaii.

Inter- and intra-site comparisons of the nitrogen (N) stable isotope composition of wetland plant species have been used to identify sources of N in coastal areas. In this study, we compared delta(15)N values from different herbaceous wetland plants across 34 different coastal wetlands from the five main Hawaiian Islands and investigated relationships of delta(15)N with land use, human population density, and surface water quality parameters (i.e., nitrate, ammonium, and total dissolved N). The highest delta(15)N values were observed in plants from wetlands on the islands of Oahu (8.7-14.6 per thousand) and Maui (8.9-9.2 per thousand), whereas plants from wetlands on the islands of Kauai, Hawaii, and Molokai had delta(15)N values usually <4 per thousand. The enrichment in delta(15)N values in plant tissues from wetlands on Oahu and Maui was most likely a result of the more developed and densely populated watersheds on these two islands. Urban development within a 1000-m radius and population density were positively correlated to average delta(15)N vegetation values from each wetland site (r = 0.56 and 0.51, respectively; p < 0.001). This suggested that site mean delta(15)N values from mixed stands of wetland plants have potential as indices of N sources in coastal lowland wetlands in Hawaii and that certain sites on Oahu and Maui have experienced significant anthropogenic N loading. This information can be used to monitor future changes in N inputs to coastal wetlands throughout Hawaii and the Pacific.

Modeled Effects of Climate Change and Plant Invasion on Watershed Function Across a Steep Tropical Rainfall Gradient

ABSTRACTClimate change is anticipated to affect freshwater resources, but baseline data on the functioning of tropical watersheds is lacking, limiting efforts that seek to predict how watershed processes, water supply, and streamflow respond to anticipated changes in climate and vegetation change, and to management. To address this data gap, we applied the distributed hydrology soil vegetation model (DHSVM) across 88 watersheds spanning a highly constrained, 4500 mm mean annual rainfall (MAR) gradient on Hawai‘i Island to quantify stream flow at 3-h time-steps for eight years in response to the independent and interactive effects of (1) large observed decrease in MAR; (2) projected warming and altered precipitation; and (3) four scenarios of forest invasion by the high water-demanding non-native tree species Psidium cattleianum. The model captured 62% of variability in measured flow at daily time scales, 95% at monthly time scales, and 98% at annual time scales. We found that low DHSVM modeled flow (Q90) and storm flow (Q10) responses to observed declines in rainfall dwarfed those of projected temperature increase or invasion, with flow decline positively correlated with MAR. As a percentage of streamflow, temperature and invasion reductions were negatively correlated with MAR. By comparison, warming alone had little effect on Q90 or Q10, but both decreased with increasing P. cattleianum cover, and projected effects of declining MAR were accentuated when combined with P. cattleianum and warming. Restoration mitigated some effects of climate warming by increasing stream base flows, with the relative effects of restoration being larger in drier versus wetter watersheds. We conclude that potential changes in climate in tropical environments are likely to exert significant effects on streamflow, but managing vegetation can provide mitigating benefits.

Biodiversity and ecosystem risks arising from using guppies to control mosquitoes

Deploying mosquito predators such as the guppy (Poecilia reticulata) into bodies of water where mosquitoes breed is a common strategy for limiting the spread of disease-carrying mosquitoes. Here, we draw on studies from epidemiology, conservation, ecology and evolution to show that the evidence for the effectiveness of guppies in controlling mosquitoes is weak, that the chances of accidental guppy introduction into local ecosystems are large, and that guppies can easily establish populations and damage these aquatic ecosystems. We highlight several knowledge and implementation gaps, and urge that this approach is either abandoned in favour of more effective strategies or that it is used much more rigorously. Controlling mosquitoes does not need to come at the expense of freshwater biodiversity.

Community Structure and Abundance of Benthic Infaunal Invertebrates in Maine Fringing Marsh Ecosystems

Fringing marshes are abundant ecosystems that dominate the New England coastline. Despite their abundance, very little baseline data is available from them and few studies have documented the ecosystems services that they provide. This information is important for conservation efforts as well as for an increased understanding of how fringing marshes function compared to larger marsh meadow systems. Benthic infaunal invertebrates were sampled from cores collected from Spartina alterniflora-dominated low marsh, Spartina patens-dominated high marsh, and Phragmites australis-invaded high marsh zones of nine fringing marsh ecosystems in Casco Bay, Maine, USA. Infaunal densities and biomass were generally higher in low marsh than high marsh or P. australis cores. Invertebrate community structure was significantly different between low marsh and high marsh and P. australis cores, which was attributed to significantly higher pore water salinity, lower organic matter, total plant percent cover, and S. patens cover in low marsh zones. There were no differences in invertebrate densities, biomass, or community structure when high marsh and P. australis cores were compared. Invertebrate densities and community structure were dominated by oligochaetes in all zones. Oligochaetes were also an important component of infaunal biomass, but the less abundant and larger invertebrates such as green crabs, tanaids, and bivalves were also large contributors to biomass in the low marsh zone. Low marsh invertebrate communities were characterized by significantly higher densities of nematodes, Nereis virens, an unidentified oligochaete, the bivalves Gemma gemma and Mya arenaria, and Leptochelia rapax. High marsh invertebrate communities were characterized by higher densities of insects, specifically Culicoides sp. ceratopogonid larvae and Anurida maritima, as well as an unidentified species of mite. Our results revealed a diverse and abundant infaunal invertebrate community that likely supports similar ecosystem services in fringing marshes as invertebrates in larger marsh meadows.

Identifying natural catchment landscape influences on tropical stream organisms: classifying stream reaches of the Hawaiian Islands

Stream classifications can be used to understand patterns within and across river networks and are most informative when they offer insight into patterns in stream habitat or biology. We developed a classification of Hawaiian stream reaches based on influences of natural landscape features on distributions of stream organisms to understand patterns in ecological potential across five Hawaiian Islands. Our objectives were to (1) identify natural landscape variables strongly associated with species distributions and likely to affect stream habitat; and (2) classify Hawaiian stream reaches based on relationships between landscape variables and distributions of native stream taxa. We used canonical correspondence analysis to identify natural landscape variables associated with distributions of nine native stream taxa. To classify reaches, we then used a conditional inference tree that identified significant influences of natural landscape variables on taxa distributions and showed that elevation, channel slope, hydrologic soil grouping, and rainfall were all important predictors of species distributions. Results were used to develop reach classes that describe differences in stream habitat. Our research adds to current understanding of landscape controls on the biota of tropical island streams and provides a tool for decision makers tasked with developing conservation and adaptation strategies.

Dedication: Dr. Michele L. Dionne (1954–2012)

In July 2012, the estuarine and coastal research and conservation community lost one of its greatest champions with the passing of Dr. Michele L. Dionne. Throughout her career, Michele’s contribution to coastal conservation extended well beyond her insights into research. She was tireless in her campaign to advance our understanding of salt marsh ecology, conservation, and restoration. Her love of coastal systemsmade her a visionary and a fighter. She was unselfish. When she did something, it was for her community, her students, or the coastal ecosystems she was charged to protect and conserve. She had the ability to think outside of the box, to see the forest through the trees or the nekton through the mummichogs. Michele began her scientific journey by receiving a Bachelor of Arts in Biology from Bates College in 1977. She then went on to get her Master of Science in Zoology from the University of North Carolina at Chapel Hill in 1982 where she studied the importance of cannibalism in fish populations. She continued her graduate studies by receiving a PhD in Biology from Dartmouth College in 1991 where she investigated habitat complexity and fish behavior. After graduating from Dartmouth College, Michele went on to become the first scientist and Director of Research at the newly created Wells National Estuarine Research Reserve (WNERR). During her tenure as the Director of Research at WNERR, she transformed the research capabilities of the reserve by generating millions of external research funds that went into building the Coastal Ecology Center that serves to house and provide modern laboratory facilities for internal research and education, external research scientists, and graduate students. This facility has and continues to nurture cadres of future ecologists and conservationists, as well as provides the infrastructure needed for well-established scientists to continue doing research in marshes and other coastal ecosystems throughout NewEngland. Her efforts also went into garnering a trust that provides funds for a post-doctoral fellow to perform high-level research in conservation ecology. Michele, her students, and post-doctoral fellows performed a variety of research activities that included studying the utilization of marsh ecosystems by fish (Dionne et al. 1999; MacKenzie and Dionne 2008), coastal restoration (Burdick and Dionne 1994; Burdick et al. 1997; Koniski et al. 2006), assessing human and climate threats on coastal systems (Chen et al. 2009; Crain et al. 2009; Dijkstra et al. 2013; Eberhardt et al. 2011), and investigating species-specific interactions in marsh ecosystems (Dijkstra et al. 2012; Tyrrell et al. 2012). She was a proponent of long-term monitoring that could provide effective baseline data sets to determine the conditions of these ecosystems (Neckles and Dionne 2000). Once asked if there was any part of research she didn’t find interesting. She replied with an emphatic, BNo! I enjoy all aspects of research!^. While Michele was successful in publishing papers and book chapters on marsh ecology and conservation, it was her unwavering support of undergraduate students, graduate students, and post-doctoral students that best describes the legacy she has left behind. She provided opportunities for so many students to discover and to grow; many of these students are now ecologists and conservationists at institutions across North America. She can still be seen today in the many presentations her students or colleagues give at the Coastal and Communicated by Iris C. Anderson