Digestion rates and assimilation efficiencies of siphonophores fed zooplankton prey

Digestion times and assimilation efficiencies are critical factors used in calculations of carbon and nitrogen budgets. Digestion times of natural copepod prey differed significantly among 4 genera of siphonophores (P>0.001), from a minimum of 1.6 h to a maximum of 9.6 h. Assimilation efficiencies, in contrast, were uniformly high; 87 to 94% for carbon and 90 to 96% for nitrogen. Nitrogen assimilation was consistently greater than carbon assimilation. Assimilation efficiencies calculated according to dry weight substantially underestimated assimilation of carbon and nitrogen, while calculations using ash-free dry weights and ash-free to dry weight ratios approached values for carbon assimilation. These values are appreciably higher than most of the assimilation efficiencies previously measured for a few other planktonic carnivores. These results indicate very efficient digestion of food by siphonophores in oceanic environments where prey densities are low.     

Prey, feeding rates, and asexual reproduction rates of the introduced oligohaline hydrozoan Moerisia lyonsi

Moerisia lyonsi Boulenger (Hydrozoa) medusae and benthic polyps were found at 0 to 5‰ salinity in the Choptank River subestuary of Chesapeake Bay, USA. This species was introduced to the bay at least 30 years before 1996. Medusae and polyps of M. lyonsi are very small and inconspicuous, and may occur widely, but unnoticed, in oligohaline waters of the Chesapeake Bay system and in other estuaries. Medusae consumed copepod nauplii and adults, but not barnacle nauplii, polychaete and ctenophore larvae or tintinnids, in laboratory experiments. Predation rates on copepods by medusae increased with increasing medusa diameter and prey densities. Feeding rates on copepod nauplii were higher than on adults and showed no saturation over the range of prey densities tested (1 to 64 prey l⁻¹). By contrast, predation on copepod adults was maximum (1 copepod medusa⁻¹ h⁻¹) at 32 and 64 copepods l⁻¹. Unexpectedly, M. lyonsi colonized mesocosms at the Horn Point Laboratory during the spring and summer in 4 years (1994 to 1997), and reached extremely high densities (up to 13.6 medusae l⁻¹). Densities of copepod adults and nauplii were low when medusa densities were high, and estimated predation effects suggested that M. lyonsi predation limited copepod populations in the mesocosms. Polyps of M. lyonsi asexually produced both polyp buds and medusae. Rates of asexual reproduction increased with increasing prey availability, from an average total during a 38 d experiment of 9.5 buds polyp⁻¹ when each polyp was fed 1 copepod d⁻¹, to an average total of 146.7 buds polyp⁻¹ when fed 8 copepods d⁻¹. The maximum daily production measured was 8 polyp buds and 22 medusae polyp⁻¹. The colonizing potential of this hydrozoan is great, given the high rates of asexual reproduction, fairly wide salinity tolerance, and existence of a cyst stage. Received: 29 October 1998 / Accepted: 3 March 1999     

Diel burying by the tropical sea cucumber Holothuria scabra: effects of environmental stimuli, handling and ontogeny

Understanding concealment behaviour of marine animals is vital for population surveys and captive-release programmes. The commercially valuable sea cucumber Holothuria scabra Jaeger 1833 (Holothuroidea) can display a diel burying cycle, but is it widely predictable? Circadian burying of captive H. scabra juveniles, and both juveniles and adults in the wild, was examined in New Caledonia. Groups of ten cultured juveniles in mesh chambers in a tank were monitored for 24 h. Small juveniles (1–5 g) displayed an expected diel cycle of epibenthic foraging in the afternoon and night then burial in sediments in the morning. Burial was related significantly to both light and temperature in combination. Similar groups of juveniles were handled once or three times a day for 1 week then frequency of emergence during another week was compared to unhandled controls. Handling stress, whether occasional or frequent, significantly suppressed the frequency of their afternoon emergence from sediments for 4 days. In a coastal seagrass bed, burial and emergence of H. scabra were monitored during days of opposing tidal cycles in three seasons. Adults seldom buried during the day except in the cool season. At that site, most small hatchery-produced H. scabra juveniles were buried during most of the day, while larger juveniles showed little diurnal burying. This study underscores that the circadian behaviours of marine animals can exhibit substantial spatial variation, may be absent at certain sites or seasons, and can be mediated by a complexity of factors that vary over short timescales.     

Ammonium excretion in a temperate-reef community by a planktivorous fish,Chromis punctipinnis (Pomacentridae), and potential uptake by young giant kelp,Macrocystis pyrifera (Laminariales)

The blacksmith Chromis punctipinnis, an abundant planktivorous damselfish off southern California, USA, shelters along rocky reefs at night. While sheltered, blacksmiths excrete ammonium that could, in turn, be utilized by nearby benthic macrophytes. Laboratory experiments during the summer and fall of 1983 and 1984 indicate that ammonium excretion at night ranged from 18.1 mol h^-1 by a 8.5 g (dry) fish, to 89.1 mol h^-1 by a 27.3 g fish; excretion rates generally declined throughout the night. Field measurements at night indicate that ammonium concentrations were significantly higher in rocky crevices occupied by blacksmiths than in unoccupied shelters, and the ammonium level in one shelter dropped after a blacksmith was experimentally removed. Young kelp plants (Macrocystis pyrifera) are capable of taking up ammonium at night. Ammonium levels in chambers containing both a blacksmith and a young kelp plant were significantly lower than in chambers containing only a fish, and ammonium levels dropped in ammoniumspiked chambers that contained kelp plants. Nighttime ammonium uptake rates by young kelp plants, which averaged 1.6 mol g^-1 (dry) h^-1, were only slightly lower than those during the day. Daytime excretion by blacksmiths occasionally results in elevated ammonium levels in the water column. On two of six days, ammonium concentrations in midwater foraging aggregations were slightly but significantly higher than in upcurrent controls; since blacksmiths typically aggregate at the incurrent margin of kelp beds, the ammonium is swept downcurrent and may be utilized by large M. pyrifera that extend through the water column. Thus, the activities of blacksmiths may results in the importation of extrinsic, inorganic nitrogen to primary producers on temperate reefs.     

Cultivating food as a right to the city

This paper argues that there is much to be gained when we view struggles to cultivate food in the city through the lens of Henri Lefebvre's concept of the right to the city. Lefebvre's idea helps us better perceive the radical political and ecological potential of those struggles. And in the empirical details of the struggles we can see concretely the key action in Lefebvre's concept, an action that is only abstract in his work: urban inhabitants becoming active and producing and managing space for themselves.     

Assessment Tools for Urban Catchments: Developing Stressor Gradients1

Abstract: This is the first in a series of three articles designed to establish empirically defined biological indicators and thresholds for impairment for urbanized catchments, and to describe a process by which the biological condition of waterbodies in urbanized catchments can be applied. This article describes alternative gradients of urbanization for assessing and selecting a nationally applicable biological index (article 2 – Purcell et al., this issue ) and defining the potential of biological communities within a gradient of cumulative stressors (article 3 – Paul et al. this issue ). Gradients were designed to represent the most prominent mosaic of stressors found in urban settings. A primary urban gradient was assembled based on readily obtained information of urbanization to include three broad‐scale parameters: percent urban land use/land cover, population density, and road density. This gradient was used as the standard by which alternative urban gradients, which included fine‐scale instream chemical and hydrologic parameters, were assessed. Five alternative gradients were developed to provide numerous environmental management options based on availability of data from water program resources. The urban gradients were developed with the intent that they be applied throughout the country; therefore, data from three different regions of the United States (Mid‐Atlantic, Midwest, and Pacific Coast) were used to validate the urban gradient model. Our study showed that a relatively straightforward stressor gradient consisting of human population density, road density, and urban land use is useful in providing a framework for developing relevant biological indicators and evaluating the potential of biological communities as a basis for assessing attainment of designated aquatic life use.     

Linking Theory and Practice for Restoration of Step-Pool Streams

Step-pools sequences are increasingly used to restore stream channels. This increase corresponds to significant advances in theory for step-pools in recent years. The need for step-pools in stream restoration arises as urban development encroaches into steep terrain in response to population pressures, as stream channels in lower-gradient areas require stabilization due to hydrological alterations associated with land-use changes, and as step-pools are recognized for their potential to enhance stream habitats. Despite an increasingly voluminous literature and great demand for restoration using step-pool sequences, however, the link between theory and practice is limited. In this article, we present four unique cases of stream restoration using step-pools, including the evolution of the approaches, the project designs, and adjustments in the system following restoration. Baxter Creek in El Cerrito, California demonstrates an early application of artificial step-pools in which natural adjustments occurred toward geomorphic stability and ecological improvement. Restoration of East Alamo Creek in a large residential development near San Ramon, California illustrates an example of step-pools increasingly used in locations where such a channel form would not naturally occur. Construction of a step-pool channel in Karnowsky Creek within the Siuslaw National Forest, Oregon overcame constraints posed by access and the type and availability of materials; the placement of logs allowed natural scouring below steps. Dry Canyon Creek on the property of the Mountains Restoration Trust in Calabasas, California afforded a somewhat experimental approach to designing step-pools, allowing observation and learning in the future. These cases demonstrate how theories and relationships developed for step-pool sequences over the past two decades have been applied in real-world settings. The lessons from these examples enable us to develop considerations useful for deriving an appropriate course of design, approval, and construction of artificial step-pool systems. They also raise additional fundamental questions concerning appropriate strategies for restoration of step-pool streams. Outstanding challenges are highlighted as opportunities for continuing theoretical work.