Seasonal variability of climate variables in Unguja Island, Zanzibar (Tanzania).

<p>(A) Average monthly wind speed (m/s). (B) Average monthly air temperature (°C). (C) Average daily precipitation (mm). Grey areas represent the standard deviations. Black lines represent average values for the period 2000–2015. Blue points represent the average monthly values during the sampling period of the present study: short rains (Nov-Dec 2014), long rains (Mar-Apr 2015), and dry season (Jul-Aug 2015). Climate data were obtained from NOAA/NCDC, available at <a href="http://gis.ncdc.noaa.gov/maps" target="_blank">http://gis.ncdc.noaa.gov/maps</a>.</p

Effects of seasonality and management on fish community structure.

<p>Results from the CAP analysis with season and management as constraints. Ellipsoids represent the minimal area that encloses all points in the group. Ellipsoids in colour represent management level (MPAs: green; Open-access sites: orange), and the empty ellipsoids represent season (dashed, light grey: short rains; dotted, grey: long rains; solid, dark grey: dry season). Dots represent individual sites and are coloured by management level and season (see figure legend).</p

Effects of seasonality and management on fish densities.

<p>Conditional effect plots of linear mixed models for (A) total fish density (square root), (B) Shannon Index, (C) species richness, (D) juvenile fish (square root), (E) subadult fish (square root) and (F) adult fish density (square root), predicted by the interaction between management and season. Black lines represent the mean value and grey areas are 95% confidence intervals. Dots represent partial residuals (coloured by site, see figure legend). SR = short rains; LR = long rains; D = dry season. Significance is marked with letters (a and b) above each figure (n.s. indicates not significant).</p

Metrics of temporal dissimilarity.

<p>Conditional effect plots of linear mixed models for the estimates of four different measures of temporal dissimilarity, extracted from the nMDS plots in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0183999#pone.0183999.g007" target="_blank">Fig 7</a>, predicted by level of management for total, juvenile, subadult and adult fish communities. Error bars indicate the 95% confidence intervals. Lines are coloured by management level (MPAs: green, Open-access sites: orange) Non-significant differences (P > 0.05) are represented by dashed lines.</p

Marine protected areas increase temporal stability of community structure, but not density or diversity, of tropical seagrass fish communities

<div><p>Marine protected areas (MPAs) have been shown to increase long-term temporal stability of fish communities and enhance ecosystem resilience to anthropogenic disturbance. Yet, the potential ability of MPAs to buffer effects of environmental variability at shorter time scales remains widely unknown. In the tropics, the yearly monsoon cycle is a major natural force affecting marine organisms in tropical regions, and its timing and severity are predicted to change over the coming century, with potentially severe effects on marine organisms, ecosystems and ecosystem services. Here, we assessed the ability of MPAs to buffer effects of monsoon seasonality on seagrass-associated fish communities, using a field survey in two MPAs (no-take zones) and two unprotected (open-access) sites around Zanzibar (Tanzania). We assessed the temporal stability of fish density and community structure within and outside MPAs during three monsoon seasons in 2014–2015, and investigated several possible mechanisms that could regulate temporal stability. Our results show that MPAs did not affect fish density and diversity, but that juvenile fish densities were temporally more stable within MPAs. Second, fish community structure was more stable within MPAs for juvenile and adult fish, but not for subadult fish or the total fish community. Third, the observed effects may be due to a combination of direct and indirect (seagrass-mediated) effects of seasonality and, potentially, fluctuating fishing pressure outside MPAs. In summary, these MPAs may not have the ability to enhance fish density and diversity and to buffer effects of monsoon seasonality on the whole fish community. However, they may increase the temporal stability of certain groups, such as juvenile fish. Consequently, our results question whether MPAs play a general role in the maintenance of biodiversity and ecosystem functioning under changing environmental conditions in tropical seagrass fish communities.</p></div

Community structure per season and management level based on fish families.

<p>Percentage number of (A) juvenile, (B) subadult, and (C) adult fish belonging to each family in MPAs and open-access areas during each season.</p

Map of Unguja Island, Zanzibar (Tanzania) and sampling locations.

<p>(A) Mnemba Island. (B) Changuu Island. (C) Chumbe Island. The green areas depict the MPAs, and the orange circles mark the four sampling sites. The blue arrows in the main map indicate the seasonal winds: dark blue is the north-easterly winds (Nov-Feb) and light blue the south-easterly winds (Apr-Sep). Background maps obtained from OpenStreetMap contributors (2016), available under the Open Database License (ODbL) at <a href="http://www.openstreetmap.org" target="_blank">www.openstreetmap.org</a>.</p

nMDS plots displaying community structure in MPA and open-access sites.

<p>(A) Total fish, (B) juveniles, (C) subadults, and (D) adult fish. Dots represent the community structure at each season, with seasons distinguished by shading (light: short rains; medium: long rains; dark: dry season). Each colour represents a particular site (see figure legend).</p

Direct and indirect (habitat-mediated) effects of seasonality on fish densities.

<p>Piecewise SEM exploring the effects of seasonality on fish densities in MPAs and open-access sites. (A) Model for total, juvenile, sub-adult and adult fish densities in MPAs; (B) model for total fish density in open-access sites; (C) models for juvenile fish density in open-access sites; (D) model for sub-adult fish density in open-access sites; and (E) model for adult fish density in open-access sites. Black arrows represent unidirectional relationship among variables (<i>P</i> ≤ 0.05), and their thickness has been scaled based on the magnitude of the standardized regression coefficient, given next to the arrows. Grey dashed arrows denote non-significant paths (<i>P</i> ≥ 0.05). R² value for each component of the model is reported in the boxes of response variables as the conditional R², which considers the effects of both fixed and random factors.</p

Community structure per season and management level based on functional groups.

<p>Percentage number of fish classified into functional groups based on feeding guilds in MPAs and open-access areas during each season.</p