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Abstract EUTROPHICATION AND FISHERIES: A GLOBAL PERSPECTIVE. Denise Breitburg, Darryl Hondorp and Lori Davias Both nitrogen loadings and the number of coastal systems experiencing
hypoxia have increased worldwide. Numerous studies have shown the
potential for negative effects of hypoxia, but scaling up from effects
at the local or individual scale to population, system-wide, and
fisheries effects is not straightforward for mobile species. Cross-system
comparisons of >35 estuaries and semi-enclosed seas in industrialized
nations suggest that the relationship between nitrogen loading and
fisheries landings is unaffected by the spatial extent of hypoxia.
N loading and fisheries landings were positively related up to about
15,000 kg N km-2 y-1, the point represented
by Chesapeake Bay. The positive relationship between N and landings
of mobile demersal species was unaffected by hypoxic extent. The
increased demersal:pelagic ratio in eutrophic systems reported in
other studies is highly dependent on fisheries regulations, the
increased catch of pelagics in some highly enriched systems, and
a very high pelagic:demersal ratio in a single system – the
Black Sea. Mean trophic level of catch and mean size of species
in the fishery also did not differ between systems with and without
extensive hypoxic or anoxic areas. Nutrient enrichment creates a
spatial mosaic of prey-enriched and physiologically stressful habitats.
Spatial averaging of enriched and degraded habitats, and preferential
use of enriched habitat, may reduce system-wide negative effects.
Turbidity may reduce piscivore capture success as well as the abundance
of macrophytes that provide a predation refuge. Fisheries exploitation
also keeps most species below carrying capacity, potentially reducing
the realized consequences of habitat loss. Our analyses suggest
that improving water quality is likely to increase populations and
fisheries landings only at the local scale and for particularly
susceptible species. Such improvements may be especially critical
in developing countries where discharge of raw sewage creates more
severe and long-lasting oxygen depletion and human populations are
more dependent on local resources. GELATINOUS ZOOPLANKTON IN A CHANGING CHESAPEAKE BAY – FOOD WEB AND LANDSCAPE STRUCTURE Denise Breitburg, Rebecca Burrell and Sarah Kolesar The scyphomedusa Chrysaora quinquecirrha and lobate ctenophore
Mnemiopsis leidyi are dominant consumers in the Chesapeake
Bay. These species are at the nexus of overlapping intraguild predation
webs. Mnemiopsis-dominated locations and time periods are
characterized by unstable dynamics and local elimination of zooplankton
and ichthyoplankton prey, while Chrysaora dominance fosters
persistence of non-gelatinous food web components. Spatial differences
in reproduction and overwintering locations lead to successive waves
of spread of these two gelatinous species in opposing directions
across the landscape. Ctenophores appear first in the main channels
of Chesapeake tributaries. They then rapidly increase in abundance
in shallow tributaries and coves and spread through the landscape.
In contrast, Chrysaora ephyra are most abundant in small
coves and tributaries, with densities spreading outward from these
small systems to the main channels of rivers as summer progresses.
As Chrysaora increases and spreads, it reduces or eliminates
ctenophores by direct predation, and by reducing ctenophore reproduction
through partial predation. Because of fairly subtle differences
in diets, these shifting temporal and spatial patterns of medusa
and ctenophore dominance potentially influence spatial distributions
of survival of fish and oyster larvae. This predator-mediated landscape
structure has likely changed over time. Historical data suggest
that the decline in Chesapeake oysters has led to a sharp decline
in sea nettle abundances. The reduction in hard substrate provided
to Chrysaora polyps by oysters appears to have caused major
changes in the magnitude and timing of trophic interactions involving
gelatinous zooplankton, copepods and early life stages of fishes,
with potential indirect influences throughout the Chesapeake Bay
food web. Most important, the reduction in sea nettles has allowed
the ctenophore to escape control by its predator and has increased
the magnitude and duration of the ctenophore’s dominance as
a consumer of copepods, fish eggs and fish larvae.
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