A meta-analysis reveals edge effects within marine protected areas.

Marine protected areas (MPAs) play a leading role in conserving and restoring marine environments. MPAs can benefit both marine populations within their boundaries and external populations owing to a net export of organisms (spillover). However, little is known about variation in performance within MPAs. For example, edge effects may degrade populations within MPAs close to their boundaries. Here we synthesize empirical estimates of 72 taxa of fish and invertebrates to explore spatial patterns across the borders of 27 no-take MPAs. We show that there is a prominent and consistent edge effect that extends approximately 1 km within the MPA, in which population sizes on the border are 60% smaller than those in the core area. Our analysis of cross-boundary population trends suggests that, globally, the smallest 64% of no-take MPAs (those of less than 10 km2 in area) may hold only about half (45-56%) of the population size that is implied by their area. MPAs with buffer zones did not display edge effects, suggesting that extending no-take areas beyond the target habitats and managing fishing activities around MPA borders are critical for boosting MPA performance.

Dinitrogen fixation in aphotic oxygenated marine environments.

We measured N2 fixation rates from oceanic zones that have traditionally been ignored as sources of biological N2 fixation; the aphotic, fully oxygenated, nitrate (NO(-) 3)-rich, waters of the oligotrophic Levantine Basin (LB) and the Gulf of Aqaba (GA). N2 fixation rates measured from pelagic aphotic waters to depths up to 720 m, during the mixed and stratified periods, ranged from 0.01 nmol N L(-1) d(-1) to 0.38 nmol N L(-1) d(-1). N2 fixation rates correlated significantly with bacterial productivity and heterotrophic diazotrophs were identified from aphotic as well as photic depths. Dissolved free amino acid amendments to whole water from the GA enhanced bacterial productivity by 2-3.5 fold and N2 fixation rates by ~2-fold in samples collected from aphotic depths while in amendments to water from photic depths bacterial productivity increased 2-6 fold while N2 fixation rates increased by a factor of 2 to 4 illustrating that both BP and heterotrophic N2 fixation were carbon limited. Experimental manipulations of aphotic waters from the LB demonstrated a significant positive correlation between transparent exopolymeric particle (TEP) concentrations and N2 fixation rates. This suggests that sinking organic material and high carbon (C): nitrogen (N) micro-environments (such as TEP-based aggregates or marine snow) could support high heterotrophic N2 fixation rates in oxygenated surface waters and in the aphotic zones. Indeed, our calculations show that aphotic N2 fixation accounted for 37 to 75% of the total daily integrated N2 fixation rates at both locations in the Mediterranean and Red Seas with rates equal or greater to those measured from the photic layers. Moreover, our results indicate that that while N2 fixation may be limited in the surface waters, aphotic, pelagic N2 fixation may contribute significantly to new N inputs in other oligotrophic basins, yet it is currently not included in regional or global N budgets.