HOW TO DEFINE? LET'S ASK NOAA and USCS WHAT ENDANGERED, THREATENED, IMPERILED, and AT-RISK MEAN

First let's see what NOAA has to say...

OCEAN SERVICE : NOAA GOV THREATENED and ENDANGERED SPECIES

Excerpt: The ENDANGERED SPECIES ACT (ESA) defines an endangered species as "any species which is in danger of extinction throughout all or a significant portion of its range." Endangered species are automatically protected by prohibitions of several types of "take," including harming, harassing, collecting, or killing, under Section 9 of the ESA. There are some limited exceptions to these rules listed in Section 10 of the ESA. The Kemp's ridley turtle, considered the smallest marine turtle in the world, is listed as an endangered species throughout its range of the Gulf of America and entire U.S. Atlantic seaboard.

The ESA defines a threatened species as "any species which is likely to become an endangered species within the foreseeable future throughout all or a significant portion of its range." Threatened species receive protections through separate regulations issued under Section 4(d) of the ESA. These regulations occur separately from the listing and detail what take prohibitions are in effect. Also called 4(d) rules, they can include the same prohibitions under Section 9. 

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USGS : ENDANGERED, THREATENED, AT RISK SPECIES

Excerpt: Under the ENDANGERED SPECIES ACT (ESA), plant and animal species may be listed as either endangered or threatened. “Endangered” means a species is in danger of extinction throughout all or a significant portion of its range. “Threatened” means a species is likely to become endangered within the foreseeable future.

States have their own ESA-type laws, so species can have different Threatened/Endangered statuses at the federal and state levels. The USGS typically refers to the federal status unless otherwise noted.

"Imperiled" or "at risk" are not legal terms under ESA, but more biological terms. Generally speaking, they are animals and plants that are in decline and may be in danger of extinction. Those terms can include species that are at low populations and near extinction but still not legally protected under ESA.

HOW DOES ANTARCTICA's BLUE ICEBERGS FORM?

EROS USGS  Earth Resources Observation and Science (EROS) Center

Excerpt:

The ice appears blue as a result of the absorption of solar radiation at yellow and red wavelengths. Deeply penetrating light is uniformly scattered at blue wavelengths by the enclosed air bubbles.

Blue ice areas are also noteworthy for their role in preserving fallen meteorites. Over time, the ice has trapped and preserved asteroid fragments, and scientists frequently scour these areas to collect them.

PACIFIC CORAL REEFS from the UNITED STATES GEOLOGICAL SURVEY

USGS - COASTAL CORAL REEFS 

Excerpts;

Coral reefs are unique ecosystems of plants, animals, and their associated geological framework.  Although they cover less that 0.5 of Earth's surface, coral reefs are home to an estimated 25% of all marine species.  Of the total coral reef habitat in the United States waters, about 75% or more than 16,000 square kilometers (6,000 square miles) is located in the Pacific Ocean.

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This site has hard science and wonderful photos of coral reef ecosystems...  See what healthy coral looks like!

SHORELINE MODELING REVEALS SOUTHERN CALIFORNIA BEACHES DISAPPEARING

USGS - SHORELINE MODELING REVEALS DISAPPEARING BEACHES

EXCERPT: Release Date: MARCH 27, 2017

Using a newly-developed computer model called “CoSMoS-COAST” (Coastal Storm Modeling System – Coastal One-line Assimilated Simulation Tool) scientists predict that with limited human intervention, 31 to 67 percent of Southern California beaches may become completely eroded (up to existing coastal infrastructure or sea-cliffs) by the year 2100 under scenarios of sea-level rise of one to two meters.