David W. Garton

Flexible energy allocation in zebra mussels (Dreissena polymorpha) in response to different environmental conditions

The ability to adjust physiological parameters in response to environmental conditions while simultaneously maintaining growth and reproduction increases an organism’s fitness. We altered energy demands to examine the effect of environmental conditions on energy allocation in zebra mussels (Dreissena polymorpha). We manipulated metabolic costs and ingestion with combinations of 4 temperatures (12, 18, 24, and 30°C), 3 rations, and 2 diet qualities for 7 wk in laboratory experiments. We measured maintenance costs (oxygen consumption and ammonia excretion), somatic tissue mass, shell growth, and reproduction to determine if zebra mussels have flexible energy allocation into tissue mass and reproduction. Increased temperature and ration increased metabolic costs but diet qualities did not. Assimilation efficiency, higher in good diet than poor, decreased with ration. Shell growth, tissue mass, and reproduction responded similarly to temperature and ration. Reproduction was similar, whereas growth and survival differed the most between diets. Good diet mussels reproduced, maintained tissue mass, grew, and survived. Poor diet mussels reproduced but did not grow and died. Diet quality influenced reproductive effort, with lower investment in body mass in the poor diet yielding higher reproductive effort. Thus, zebra mussels are flexible in energy allocation and in stressful conditions, most importantly reduced food quality, reproduce at the expense of maintenance thereby increasing the probability of death.

Shoreline Changes and Sediment Redistribution at Palmyra Atoll (Equatorial Pacific Ocean): 1874–Present

Abstract Reef islands of Palmyra Atoll in the northern Line Islands, equatorial Pacific Ocean, underwent mainly minor natural changes in shape and size between 1874 and 1940. Many major changes occurred between 1940 and 1945 when military construction created a dredged channel into the lagoon, enlarged several islands, joined most islands around the lagoon system into a continuous roadway, constructed a causeway separating two lagoons, and created several new islands. Overall, land area approximately doubled and land volume approximately trebled during this period, and the construction affected water circulation between reef flats and lagoons. Since 1945, the atoll has been largely uninhabited, construction and most maintenance of the newly-created shoreline has ceased, and coastlines of larger islands have simplified by infilling of bays and erosion of promontories, at net rates of up to 1.8 m/y. Narrower land masses have been broken into multiple smaller arcuate islands with intervening shallow channels. Sediment moves mainly westward along ocean coasts, into lagoons on lagoon coasts, and via tidal currents where causeways have been breached. In the east, Barren Island appeared and began growing from natural causes well before military construction. After purchase by The Nature Conservancy, the atoll was designated a U.S. National Wildlife Refuge in 2001. This protection offers the opportunity to observe coastal processes operating in the absence of the efforts to protect infrastructure typically associated with inhabited atolls.

Habitat shift in invading species: Zebra and quagga mussel population characteristics on shallow soft substrates

Unexpected habitat innovations among invading species are illustrated by the expansion of dreissenid mussels across sedimentary environments in shallow water unlike the hard substrates where they are conventionally known. In this note, records of population characteristics of invading zebra (Dreissena polymorpha) and quagga (Dreissena bugensis) mussels from 1994 through 1998 are reported from shallow (less than 20 m) sedimentary habitats in western Lake Erie. Haphazard SCUBA collections of these invading species indicated that combined densities of zebra and quagga mussels ranged from 0 to 32,500 individuals per square meter between 1994 and 1998, with D. polymorpha comprising 75–100% of the assemblages. These mixed mussel populations, which were attached by byssal threads to each other and underlying sand-grain sediments, had size–frequency distributions that were typical of colonizing populations on hard substrates. Moreover, the presence of two mussel cohorts within the 1994 samples indicated that these species began expanding onto soft substrates not later than 1992, within 4 years of their initial invasion in western Lake Erie. Such historical data provide baselines for interpreting adaptive innovations, ecological interactions and habitat shifts among the two invading dreissenid mussel species in North America.

VARIATION IN GENOTYPE FREQUENCIES DURING THE LIFE HISTORY OF THE BIVALVE, DREISSENA POLYMORPHA

’Address for correspondence. larva. Gametes are shed into the water where fertilization occurs, and larvae remain in the plankton for approximately 10 d before settling (Walz 1978; Lewandowski 1982b). The Great Lakes population appears to be derived from a single founding group (Griffiths et al. 1991); thus, migration from other, genetically distinct populations can be discounted as an explanation for any genetic differences observed among life history stages. Furthermore, D. polymorpha is the only mollusk in most of the Great Lakes with a planktonic larval stage thus, problems associated with identification of larvae are avoided. We examined allozyme frequencies throughout the life history of D. polymorpha in the Great Lakes in order to directly compare the genetic structure of the planktonic larvae with that of benthic stages.

The atmospheric lead record preserved in lagoon sediments at a remote equatorial Pacific location: Palmyra Atoll, northern Line Islands

Anthropogenic lead (Pb) inputs to the atmosphere increased greatly over the past century and now dominate Pb supply to the oceans. However, the Pb content of sediments across the equatorial Pacific region is relatively unknown, and data exist only for deep sea sites where Pb deposition lags surface water inputs by up to a century. Here we present ICP-MS analyses of Pb of a core from a lagoon at Palmyra Atoll, northern Line Islands, that spans approximately the past 160 years. The non-bioturbated sediments of the euxinic lagoon, coupled with rapid rates of deposition, provide a unique fine-scale record of atmospheric Pb supply at a remote Pacific location. These first observations of historic Pb sedimentation in an atoll lagoon reveal a 63-fold increase in Pb flux to sediments during the past century and correlate directly with the North American consumption of leaded gasoline that began in 1926.

Distant Storms as Drivers of Environmental Change at Pacific Atolls

The central Pacific Ocean with its many low lying islands and atolls is under threat from sea level rise and increased storm activity. Here, we illustrate how increasing frequency and severity of large scale storm events associated with global climate change may be particularly profound at the local scale for human populations that rely on lagoon systems for provision of a variety of goods and services. In August 2011 a storm originating in the Southern Ocean caused a large amplitude ocean swell to move northward through the Pacific Ocean. Its arrival at Palmyra Atoll coincided with transient elevated sea surface height and triggered turnover of the lagoon water column. This storm-induced change to the lagoon reflects long distance connectivity with propagated wave energy from the Southern Ocean and illustrates the increasing threats generated by climate change that are faced by human populations on most low-lying Pacific islands and atolls.

Conservation management options and actions: Putative decline of coral cover at Palmyra Atoll, Northern Line Islands, as a case study

Localised loss of live coral cover at Palmyra Atoll (central Pacific Ocean) has been attributed to increased temperature and/or sedimentation arising from alterations made to the lagoon system. It has been hypothesised that a causeway spanning the lagoon hinders water circulation, resulting in warmer and/or more turbid water flowing towards a site of high coral cover and diversity (Coral Gardens). Analyses of a multi-site and multi-year data set revealed no differences in mean temperature or turbidity values on either side of the causeway and provided no evidence of significantly warmer or more turbid water at Coral Gardens. We conclude that the putative decline in live coral cover cannot be attributed to the presence of the causeway and that proposed management actions involving modification to the causeway cannot achieve the conservation outcomes suggested of them.